## Examen

# Chamberlain NR503 Epidemiology Final Exam ( Version 2) / Chamberlain NR 503 Final Exam (New 2020): Population Health, Epidemiology & Statistical Principles | 100 % VERIFIED ANSWERS, GRADE A

NR 503 Epidemiology Final Exam / NR503 Epidemiology Final Exam (Latest): Chamberlain Chamberlain NR 503 Epidemiology Final Exam / Chamberlain NR503 Final Exam: Population Health, Epidemiology & Statistical Principles 1. Which of the following is a condition which may occur during the incubation period? a. Onset of clinical illness b. Receipt of infection c. Signs & symptoms of disease d. Transmission of infection e. Isolation of disease carrier through quarantine 2. Chicken pox is a highly communicable disease. It may be transmitted by direct contact with a person infected with the varicella-zoster virus (VZV). The typical incubation time is between 10 to 20 days. A boy started school 2 weeks after showing symptoms of chicken pox including mild fever, skin rash, & fluid-filled blisters. One month after the boy returned to school, none of his classmates had been infected by VZV. The main reason was: a. Herd immunity b. All had been immunized prior to the school year c. Contact was after infectious period d. Subclinical infections were not yet detected e. Disease was endemic in the class 3. Which of the following is characteristic of a single-exposure, common-vehicle outbreak? a. Long latency period before many illnesses develop b. There is an exponential increase in secondary cases following initial exposures c. Cases include only those who have been exposed to sick persons d. The epidemic curve has a normal distribution when plotted against the logarithm of time e. Wide range in incubation times for sick individuals 4. What is the diarrhea attack rate in persons who ate both ice cream & pizza? a. 39/52 b. 21/70 c. 39/67 d. 51/67 e. None of the above 5. What is the overall attack rate in persons who did not eat ice cream? a. 30% b. 33% c. 35% d. 44% e. 58% 6. Which of the food items (or combination of items) is most likely to be the infective item(s)? a. Pizza only b. Ice cream only c. Neither pizza or ice cream d. Both pizza & icecream e. Cannot be assumed from the data shown 7. Which of the following reasons can explain why a person who did not consume the infective food item got sick? a. They were directly exposed to persons who did eat the infective food item b. Diarrhea is a general symptom consistent with a number of illnesses c. There may have been an inaccurate recall of which foods were eaten d. All of the above e. None of the above An outbreak of gastroenteritis occurred at a boarding school with a student enrollment of 846. Fifty-seven students reported symptoms including vomiting, diarrhea, nausea, & low-grade fever between 10 p.m. on September 24 & 8 p.m. on September 25. The ill students lived in dormitories that housed 723 of the students. The table below provides information on the number of students per type of residence & the number reporting illnesses consistent with the described symptoms & onset time. Calculate the attack rate among all students at the boarding school. 1. Calculate the attack rate among all students at the boarding school. 57/846 = The answer is found by dividing the total number of cases (57) by the total number of students (846). This equals 6.7%. 10. Calculate the attack rates for boys & girls separately. a. For boys, the attack rate includes all cases (40 3) divided by the total number of students who are boys (380 46). The attack rate is 10.1%. b. For girls, the attack rate includes all cases (12 2) divided by the total number of students who are girls (343 77). The attack rate is 3.3%. 11. What is the proportion of total cases occurring in boys? Answer: The proportion of cases occurring in boys is equal to the number of cases in boys divided by the total number of cases (43/57). This equals 75.4%. 12. What is the proportion of total cases occurring in students who live in dormitories? Answer: The proportion of cases occurring in dormitory residents is equal to the number of cases in residents divided by the total number of cases (52/57). This equals 91.2%. 13. Which proportion is more informative for the purpose of the outbreak investigation? Answer: Both proportions are useful. Dormitory residents account for over 90% of the cases indicating an outbreak of an infectious agent that was transmitted at the school. Furthermore, over 75% of the cases were boys indicating that the responsible agent was more likely to have been transmitted in the boys’ dormitory. A group of researchers are interested in conducting a clinical trial to determine whether a new cholesterol-lowering agent was useful in preventing coronary heart disease (CHD). They identified 12,327 potential participants for the trial. At the initial clinical exam, 309 were discovered to have CHD. The remaining subjects entered the trial & were divided equally into the treatment & placebo groups. Of those in the treatment group, 505 developed CHD after 5 years of follow-up while 477 developed CHD during the same period in the placebo group. 14. What was the prevalence of CHD at the initial exam? Answer: The prevalence of CHD at the initial exam was 309 cases of CHD divided by 12,327 participants. This equals a prevalence of 25.1 cases of CHD per 1,000 persons. 15. What was the incidence of CHD during the 5-year study? Answer: The incidence rate reflects the number of new cases developing in the population at risk. Since prevalent CHD cases were excluded from the study, the population at risk was 12,018 (12,327 persons less 309 cases of CHD). During the 5-year study period, 982 incident cases of CHD developed. This equals an incidence rate of 81.7 cases of CHD per 1,000 persons. 16. Which of the following are examples of a population prevalence rate? a. The number of ear infections suffered by 3-year-old children in March, 2006 b. The number of persons with hypertension per 100,000 population c. The number of cases of skin cancer diagnosed in a dermatology clinic d. b & c e. All of the above Rationale: Prevalence is the number of affected persons in a specified population size at a given time. Only answer (b) fits this definition. Example (a) is more consistent with an incident rate while answer (c) is a selected group of persons who may not be representative of a general population. 17. What would be the effect on age-specific incidence rates of uterine cancer if women with hysterectomies were excluded from the denominator of incidence calculations assuming that most women who have had hysterectomies are older than 50 years of age. A. The rates in all age groups would remain the same. B. Only rates in women older than 50 years of age would tend to decrease. C. Rates in women younger than 50 years would increase compared to women older than 50 years of age. D. Rates would increase in women older than 50 years of age but may decrease in younger women as they get older. E. It cannot be determined whether the rates would increase or decrease. A survey was conducted among 1,000 r&omly sampled adult males in the United States in 2005. The results from this survey are shown below. 18. The researchers stated that there was a doubling of risk of hypertension in each age group younger than 60 years of age. You conclude that the researchers’ interpretation: a. Is correct b. Is incorrect because prevalence rates are estimated c. Is incorrect because it was based on proportions of the population sample d. Is incorrect because incidence rates do not describe risk e. Is incorrect because the calculations do not include adult females 19. The incidence & prevalence rates of a chronic childhood illness for a specific community are given below. Based on the data, which of the following interpretations best describes disease X? • The duration of disease is becoming shorter. • The duration of disease is becoming longer. • The case-fatality rate of this disease is decreasing. • Efforts to prevent new cases of this disease are becoming more successful. • The risk of the disease has decreased over the past 20 years. 20. What is the incident rate of tuberculosis per 100,000 persons in 2003? Answer: The answer is 29 new cases of tuberculosis per 100,000 persons. This is found by dividing the new cases of tuberculosis by the total population at risk (580/2,000,000) & multiplying this rate by 100,000 to st&ardize the rate. 21. Has the risk of tuberculosis increased or decreased during 2003? Answer: The risk of tuberculosis has increased over the historic incident rate. This comparison can be made by st&ardizing the historic rate to a rate per 100,000 persons. To do this, multiply the numerator & denominator by 25. 22. Which of the following is an advantage of active surveillance? A. Requires less project staff B. Is relatively inexpensive to employ C. More accurate due to reduced reporting burden for health care providers D. Relies on different disease definitions to account for all cases E. Reporting systems can be developed quickly 23. The population of a city on February 15, 2005, was 36,600. The city has a passive surveillance system that collects hospital & private physician reports of influenza cases every month. During the period between January 1 & April 1, 2005, 2,200 new cases of influenza occurred in the city. Of these cases, 775 persons were ill with influenza according to surveillance reports on April 1, 2005. The monthly incidence rate of active cases of influenza for the 3-month period was: a. 4 per 1,000 population b. 17 per 1,000 population c. 20 per 1,000 population d. 39 per 1,000 population e. 130 per 1,000 population 22. The prevalence rate of active influenza as of April 1, 2005, was: • 10 per 1,000 population • 14 per 1,000 population • 17.5 per 1,000 population • 20 per 1,000 population • Cannot be calculated as there is no information on duration 23. What can be inferred about influenza cases occurring in the city? • Active surveillance would enable better prevention of influenza • The incidence rate would decrease if active surveillance were employed • The average duration of influenza is approximately 1 month • The actual number of influenza cases occurring in the population is less since hospitals & private physicians may be reporting the same patients. • The prevalence rate should be higher since it should be calculated based on all cases of influenza occurring from January 1 through March 30, 2005. 24. A study found that adults older than age 50 had a higher prevalence of pneumonia than those who were younger than age 50. Which of the following is consistent with this finding? • Younger adults have a higher incidence of pneumonia • Older adults have a higher case-fatality rate from pneumonia • Younger adults with pneumonia are more likely to report being ill than older persons • Incidence rates do not vary by age, but older adults have pneumonia for a longer duration compared to younger adults • None of the above 25. Which of the following statements are true? More than one answer may be correct. a. Prevalence rates are always larger than incidence rates b. In a steady state, the prevalence of disease is equal to the attack rate c. Diagnostic criteria rarely impact estimates of disease prevalence & incidence d. Prevalence rates are useful for public health planning e. Incidence rates can be used to estimate prevalence when the mean duration of the disease is known 26. A disease has an incidence of 10 per 1,000 persons per year, & 80% of those affected will die within 1 year. Prior to the year 2000, only 50% of cases of the disease were detected by physician diagnosis prior to death. In the year 2000, a lab test was developed that identified 90% of cases an average of 6 months prior to symptom onset; however, the prognosis did not improve after diagnosis. Comparing the epidemiology of the disease prior to 2000 with the epidemiology of the disease after the development of the lab test, which statement is true concerning the disease in 2000? a. Incidence is higher & prevalence is higher than in 1999 b. Incidence is higher in 2000 but prevalence remains the same c. Incidence is the same in 2000 but prevalence is higher than in 1999 d. Both incidence & prevalence remain the same as in 1999 e. Incidence is the same in 2000 but prevalence is lower than in 1999 27. Which statement is true concerning the duration of the disease after the development of the lab test? • Mean duration of a case of the disease is shorter in 2000 • Mean duration of a case of the disease is the same in 2000 • Mean duration of a case of the disease is longer in 2000 • No inference about mean duration can be made since the lab test has only been available for 1 year 28. Which statement is true concerning the disease-specific mortality rate after the development of the lab test? • The mortality rate for the disease is decreased in 2000 • The mortality rate for the disease is the same in 2000 • The mortality rate for the disease is increased in 2000 • No inference about the mortality rate can be made since the lab test has only been available for 1 year 29. In a coastal area of a country in which a tsunami struck, there were 100,000 deaths in a population of 2.4 million for the year ending December 31, 2005. What was the all-cause crude mortality rate per 1,000 persons during 2005? 30. In an industrialized nation, there were 192 deaths due to lung diseases in miners ages 20 to 64 years. The expected number of deaths in this occupational group, based on age-specific death rates for lung diseases in all males ages 20 to 64 years, was 238 during 1990. What was the st&ardized mortality ratio (SMR) for lung diseases in miners? 31. In 2001, a state enacted a law that required the use of safety seats for all children under 7 years of age & m&atory seatbelt use for all persons. The table below lists the number of deaths due to motor vehicle accidents (MVAs) & the total population by age in 2000 (before the law) & in 2005 (4 years after the law was enacted). What is the age-specific mortality rate due to MVAs for children ages 0 to 18 years in 2000? Answer: 6.1 per 1,000 Rationale: The rate is found by combining the MVA deaths & total population size for the two age groups under 7 years & 7 to 18 years during the year 2000. This equals (44 105) divided by (3,500 21,000). Multiplying this rate by 1,000 persons gives the answer indicated. 32. Using the pooled total of the 2000 & 2005 populations as the st&ard rate, calculate the age-adjusted mortality rate due to MVAs in 2005. Answer: 2.3 MVA deaths per 1,000 persons. The key to calculating the age-adjusted rate is to pool the observed numbers for both time periods & to calculate the expected numbers of deaths in the 2005 population assuming that a common rate applied to the population. For example, for those under 7 years, the pooled rate equals (44 20) divided by (3,500 4,000). The pooled rate for this group is 8.5 per 1,000 persons. When this rate is multiplied by the 4,000 children under 7 years of age in 2005, the expected number of deaths is 34.13. Performing the same calculation for each age group results in 111.7 deaths in those 7 to 18 years of age, 175.8 deaths in those 19 to 49 years, & 237.35 deaths for those 50 years or more. The total number of deaths expected in 2005 based on this pooled rate is 558.98. Therefore, the age-adjusted overall rate for 2005 is 558.98 deaths divided by 240,000 persons. 33. Based on the information in the table, it was reported that there was an increased risk of death due to MVAs in the state after the law was passed. These conclusions are: Answer: Correct, because both the total & the age-adjusted mortality rates are higher in 2005 than in 2000 Rationale: The overall crude (unadjusted) mortality rate is 2.6 per 1,000 persons in 2005. This is found by dividing 640 deaths by a population of 240,000 persons. This rate is then multiplied by 1,000. The overall adjusted mortality rate is 2.3 per 1,000 persons as calculated in question 34. Both of these rates are higher than the overall crude mortality rate of 2.0 per 1,000 persons for the year 2000. 34. For colorectal cancer diagnosed at an early stage, the disease can have 5-year survival rates of greater than 80%. Which answer best describes early stage colorectal cancer? • Incidence rates & mortality rates will be similar • Mortality rates will be much higher than incidence rates • Incidence rates will be much higher than mortality rates • Incidence rates will be unrelated to mortality rates • None of the above Rationale: For diseases with a long duration as indicated by high 5-year survival rates for early stage colorectal cancer, the incidence will be much higher than the mortality rate since more persons are being diagnosed with the disease than are dying of it. . The following table gives the mean annual age-specific mortality rates from measles during the first 25 years of life in successive 5-year periods. You may assume that the population is in a steady state (i.e., migrations out are equal to migrations in). 35. The age-specific mortality rates for the cohort born in 1915-1919 are: Answer= 2.4 3.3 2.0 0.6 0.1 Rationale: This is found by tracking the cohort of children born between 1915 & 1919 by each 5-year age group. For example, this group would be 0 to 4 years of age in 1915 to 1919 with a rate of measles mortality of 2.4. In 1920 to 1924, this group of children would be 5 to 9 years of age & have a rate of measles mortality of 3.3. Continuing in a diagonal manner, the remaining three rates can be found in the table. 36. Based on the information above, one may conclude: • Children ages 5 to 9 had the highest rate of death in all periods Rationale: For each 5-year period, the highest mortality rate is reported among those 5 to 9 years of age. This is seen by comparing the rate for this age group to all other age groups in a row. 37. Which of the following characteristics indicate that mortality rates provide a reliable estimate of disease incidence? More than one answer may be correct. a. The case-fatality rate is high b. The duration of disease is short 38. Which of the following statements are true? More than one answer may be correct. Answer: A mortality rate is an example of an incidence rate Rationale: A mortality rate can approximate an incidence rate under conditions of a high case-fatality rate & a short duration of disease. 39. Among those who are 25 years of age, those who have been driving less than 5 years had 13,700 motor vehicle accidents in 1 year, while those who had been driving for more than 5 years had 21,680 motor vehicle accidents during the same time period. It was concluded from these data that 25-year-olds with more driving experience have increased accidents compared to those who started driving later. This conclusion is: Answer: incorrect because rates are not reported Rationale: The information provided only enumerates motor vehicle accidents in two groups. In order to fully compare these counts, information is needed on the denominator, i.e., the number of persons driving in each group, so that rates can be calculated. 40. For a disease such as liver cancer, which is highly fatal & of short duration, which of the following statements is true? Choose the best answer. • Mortality rates will be much higher than incidence rates • Mortality rates will be much higher than prevalence rates • Incidence rates will be much higher than mortality rates • Case-fatality rates will be equal to mortality rates • Incidence rates will be equal to mortality rates Rationale: Since the 5-year survival rate for liver cancer is 4%, most incident cases of liver cancer will result in a premature mortality. In this case, the mortality & incidence rates will be approximately equal. 41. The prevalence rate of a disease is two times greater in women than in men, but the incidence rates are the same in men & women. Which of the following statements may explain this situation? • The duration of disease is shorter in women • Men are at greater risk for developing the disease • The case-fatality rate is lower for women • The age-adjusted mortality rate will be higher for women • The proportionate mortality rate for the disease is higher for men Rationale: Since men & women develop the disease at the same rate, the survival rate in women must be increased in order to increase duration & prevalence. A low case-fatality rate would contribute to an increased duration of the disease. 42. The table below describes the number of illnesses & deaths caused by plague in four communities. The case-fatality rate associated with plague is lowest in which community? • Community A • Community B • Community C • Community D Rationale: The case-fatality rate equals the number of deaths occurring from plague divided by all persons with the plague. In Community C, the CFR is 300 divided by 400, or 60%. This is lower than A (67%), B (75%), & D (77%). 43. The table below describes the number of illnesses & deaths caused by plague in four communities.The proportionate mortality ratio associated with plague is lowest in which community? Answer: Community D Rationale:The proportionate mortality rate equals the number of deaths occurring from plague divided by all persons with the plague. In Community D, the PMR is 500 divided by 5000, or 10%. This is lower than A (50%), B (75%), & D (38%). Chapters 5-6 1. In a community-based hypertension testing program called HT-Aware, the detection level for high blood pressure is set at 140 mmHg for systolic blood pressure. A separate testing program called HT-Warning in the same community sets the level at 130 mmHg for high systolic blood pressure. Which statements are likely to be true? a. The sensitivity of HT-Warning is greater than that of HT-Aware b. The specificity of HT-Warning is greater than that of HT-Aware c. The number of false positives is greater with HT-Warning than with HT-Aware d. The number of false negatives is greater with HT-Warning than with HT-Aware e. The sensitivity & specificity are the same for both tests 2. A school nurse examined a population of 1,000 children in an attempt to detect nearsightedness. The prevalence of myopia in this population is known to be 15%. The sensitivity of the examination is 60% & its specificity is 80%. All children labeled as “positive” (i.e., suspected of having myopia) by the school nurse are sent for examination by an optometrist. The sensitivity of the optometrist’s examination is 98% & its specificity is 90%. How many children are labeled “positive” by the school nurse? a. Answer: 150 b. Rationale: There are 150 children with myopia in the school population (15% prevalence among 1,000 children). The school nurse will identify 60% of those who truly have the condition, or 90 cases (60% sensitivity multiplied by 150 myopic children). Further, the school nurse will incorrectly identify 170 false positive cases of myopia among those who do not have the condition (80% specificity multiplied by 850 non-myopic children). The sum of the cases labeled as positive by the school nurse equals 260 children (90 true myopic children plus 170 false positive children). 3. What is the positive predictive value (PPV) of the school nurse’s exam? a. The PPV of the school nurse’s exam is equal to the number of true positive cases divided by the total number of those that the school nurse labels as positive. In this exam, the PPV is 34.6% (90 true myopic children divided by 260 children labeled as myopic by the school nurse). 4. How many children will be labeled myopic following the optometrist’s exam? a. Since the optometrist will only test children who have been labeled as myopic by the school nurse, the testing group for this sequential exam is 260 children. The optometrist labels 105 children as myopic. Among the 90 myopic children correctly referred by the school nurse, the optometrist identifies 88 of them as myopic (98% sensitivity multiplied by 90 true cases of myopia). Further, the optometrist will incorrectly identify 17 false positive cases among the 170 children referred by the school nurse who do not have myopia. The sum of the cases labeled as positive by the optometrist equals 105 children (89 true cases plus 17 false positive cases). 5. What is the positive predictive value (PPV) of the optometrist’s exam? a. The PPV of the optometrist’s exam is equal to the number of true positive cases divided by the total number that the optometrist labels as positive. The optometrist will only test 260 children referred by the school nurse. Of these children, the optometrist will correctly identify 89 cases of myopia among 105 children labeled as positive for the condition. The PPV equals 83.8% (89 true myopic children divided by 105 children labeled as positive). 6. What is the negative predictive value (NPV) of the optometrist’s exam? a. The NPV of the optometrist’s exam is 98.7%. The NPV equals the number of true negative cases divided by all negative cases indicated by the exam. In this instance, the optometrist correctly identifies 153 children as negative for myopia; however, there are 2 false negative cases following the optometrist’s exam (90 true cases referred by the school nurse less the 88 cases detected by the optometrist). The NPV equals 153 divided by 155, or 98.7%. 7. What is the overall sensitivity of the sequential examinations? a. The overall sensitivity of the sequential exams is 58.7%; 88 true positive cases of myopia are found following the optometrist’s exam among the 150 prevalent cases in the school population. 8. What is the overall specificity of the sequential examinations? a. The overall specificity of the sequential exams is 98%; 833 children will be correctly labeled as negative for myopia among the 850 true negative cases. This is found by summing the number of true positives after each exam (680 following that of the school nurse plus 153 following the optometrist) & dividing by the true negative children in the population. This equals 833 divided by 850, or 98%. 9. What would be the positive predictive value (PPV) of the exam for myopia if the optometrist tested all 1,000 children? a. The PPV of the optometrist’s exam would be equal to the number of true positive cases divided by all children labeled positive by the optometrist. Applying the sensitivity & specificity of the optometrist’s exam to the 1,000 children would indicate that 147 true positive cases are labeled positive by the optometrist. Additionally, the optometrist would find 85 false positive cases (850 true negative cases multiplied by 90% specificity). The PPV would be 63.4% (147 true positive cases divided by 232 total positives indicated by the optometrist 10. Which of the following improves the reliability of diabetes screening tests? a. Having the same lab analyze all samples b. Taking more than one sample for each subject & averaging the results c. Insuring that the instrument is st&ardized before each sample is analyzed d. a & c only e. All of the above Rationale: Reliability is improved by consistency of analyses, especially when multiple samples are taken for a subject & the analytic instrument is routinely st&ardized. 11. A prostate specific antigen (PSA) test is a quick screening test for prostate cancer. A researcher wants to evaluate it using two groups. Group A consists of 1,500 men who had biopsy-proven adenocarcinoma of the prostate while group B consists of 3,000 age- & race-matched men all of whom showed no cancer at biopsy. The results of the PSA screening test in each group is shown in the table. What is the sensitivity of the PSA screening test in the combined groups? Answer: The sensitivity equals the number of true positives detected among all true positives. Since a biopsy is the gold st&ard test for prostate cancer, all 1,500 men in group A are positive for prostate cancer. The PSA test indicated that 1,155 of these men had prostate cancer, a sensitivity of 77%. 12. What is the specificity of the screening test in the combined groups? a. Answer: 85 b. Rationale: The specificity equals the number of true negatives detected among all true negatives. Among the 3,000 men who did not have prostate cancer, the test correctly identified 2,760 men as negative for prostate cancer (3,000 minus 240 false positives). This gives a sensitivity of 92%. 13. What is the positive predictive value (PPV) of the screening test in the combined groups? a. The PPV is 83%. b. rationale: This value is found by dividing 1,155 true positives by the total number of all positives indicated by the PSA test (1,155 plus 24). 14. The PSA screening test is used in the same way in two equal-sized populations of men living in different areas of the United States, but the proportion of false positives among those who have a positive PSA test in the first population is lower than that among those who have a positive PSA test in the second population. What is the likely explanation for this finding? a. It is impossible to determine what caused the difference b. The prevalence of disease is higher in the first population c. The specificity of the test is lower in the first population d. The specificity of the test is higher in the first population e. The prevalence of the disease is lower in the first population Rationale: We can assume that the specificity of the test will be similar in each population. Therefore the proportion of false positives found among the true negatives should be the same in each population. However, the proportion of false positives among all positives on the PSA screening test will be influenced by the number of true positives detected by the test. Since the sensitivity of the test will also be the same, we can assume that more true positives exist in the population of men with a lower proportion of false positive tests due to an increase in the PPV. 15. Test A has a sensitivity of 95% & a specificity of 90%. Test B has a sensitivity of 80% & a specificity of 98%. In a community of 10,000 people with 5% prevalence of the disease, Test A has always been given before Test B. What is the best reason for changing the order of the tests? a. The net sensitivity will be increased if Test B is given first b. The total number of false positives found by both tests is decreased if Test B is given first c. The net specificity will be decreased if Test B is given first d. The total number of false negatives found by both tests is decreased if Test B is given first e. There is no good reason to change the order of the tests Rationale: A sequential testing process would only refer those with positive results to the second test. Since Test B has a higher specificity, then fewer false positives will be referred for Test A, thereby decreasing the number of false positives found. This can be shown by calculation if we assume that 500 persons have the disease among the 10,000 in the population. Test B will find only 190 false positives for referral (9,500 true negatives less the number of true negatives multiplied by 98% specificity). Performing Test A first results in 950 false positives referred for the second test (9,500 true negative less the number of true negatives multiplied by 90% specificity). 16. Two neurologists, Drs. J & K, independently examined 70 magnetic resonance images (MRIs) for evidence of brain tumors. As shown in the table below, the neurologists read each MRI as either “positive” or “negative” for brain tumors. Based on the above information, the overall percent agreement between the two doctors including all observations is: a. 62.9% b. Rationale: The two doctors agree on 44 of the 70 MRI readings. This includes the 26 that they both labeled as positive for brain tumors & the 18 that they both agreed were negative for brain tumors. 17. What is the estimate of kappa for the reliability of the two doctors’ test results? a. 24.9% b. Rationale: The estimate of kappa expresses the observed agreement of two testers in excess of chance alone. It is found by applying the expected agreement rates for both testers. In this case, Dr. K labeled 38 of the 70 MRIs as positive (54.3% of all MRIs) & 32 as negative (45.7% of all slides). Dr. J labeled 57.1% of the MRIs as positive (40 of 70) & 42.9% as negative. We would expect that if Dr. K had the same rate of positive & negative findings as Dr. J then they would agree by chance on 21.7 of the 38 positive MRIs that were found (38 multiplied by 0.571). Further, they would agree by chance on 13.7 of the 32 negative MRIs that were found (32 multiplied by 0.429). Therefore, we would expect the two doctors to agree by chance on 50.6% of the MRIs (21.7 positive agreements plus 13.7 negative agreements equals 35.4, then divide this by the total of 70 to get an expected overall agreement of 50.6%). Now, kappa can be calculated as the observed agreement less expected divided by 100% less the expected agreement— in this instance, 62.9% minus 50.6% divided by 100% less 50.6%. 12.3% divided by 49.4% results in a kappa of 24.9%. 18. In the general population, the prevalence of coronary artery disease is apporximately 6%. Assuming that this sample of patients is representative of the general population, the sensitivity of the CMR test in the general population would be approximately: a. Answer: between 90 & 95% b. Rationale: If we assume that the prevalence of disease is similar, then we can accept that 60 persons with a positive x-ray will be true cases of coronary artery disease. In this instance, the CMR test positively identifies 56 of the 60 true cases, a sensitivity of 93.3%. 19. After reviewing the results of the test comparison, an epidemiologist decides that the specificity of the test is too low. Using the same CMR images, he raises the cutoff value for a positive test to increase the specificity. What is the likely effect on the sensitivity? a. Sensitivity will decrease b. Rationale: The increase in the cutoff value for a positive test will reduce the sensitivity of the test even though the specificity is increased. This will result in the misidentification of true positive cases as false negatives if their CMR values are below the cutoff value suggested by the epidemiologist. 20. In comparing the mammography readings of two technicians who evaluated the same set of 600 mammograms for presence of breast cancer from a generally representative sample of women from the population, a. Answer: Overall percent agreement calculated for both readers may conceal significant disagreements regarding positive tests b. Rationale: Since the sample is from the general population, it is likely that very few will have prevalent breast cancer indicating that many readings will be regarded as normal, or negative for the disease. Since a large proportion of the readings will be negative, it is likely that the two technicians will have a high value for overall percent agreement though they may differ significantly in their readings for the few women who are labeled positive for breast cancer. 21. In a country with a population of 16 million people, 175,000 deaths occurred during the year ending December 31, 2005. These included 45,000 deaths from tuberculosis (TB) in 135,000 persons who were sick with TB. Assume that the population remained constant throughout the year. a. What was the annual mortality rate for the country during 2005? i. The annual mortality rate equals the number of deaths divided by the total population. In this example, 175,000 deaths occurred among 16 million persons. Dividing these numbers & multiplying by 100,000 gives a rate of 1,094 deaths per 100,000 persons, approximately 1% of the population. b. What was the case-fatality rate (CFR) from TB during 2005? i. The CFR is the number of cause-specific deaths divided by all cases of the specific disease. In this example, 45,000 TB deaths occurred in 135,000 persons with TB. This equals a CFR of 33%. c. What is the proportionate mortality ratio (PMR) for TB during 2005? i. The PMR is the number of deaths due to a specific cause divided by all deaths. In this example, the PMR equals 45,000 TB deaths divided by 175,000 deaths, or approximately 26%. 22. In a country with a population of 16 million people, 175,000 deaths occurred during the year ending December 31, 2005. These included 45,000 deaths from tuberculosis (TB) in 135,000 persons who were sick with TB. Assume that the population remained constant throughout the year. Not all 135,000 cases of TB were contracted during 2005. Which of the following statements is true? a. The case-fatality rate provides a reasonable estimate of incidence b. The prevalence of TB for 2005 is equal to the denominator of the case-fatality rate c. The duration of TB is brief d. All of the above e. None of the above Rationale: Since the duration of TB can be longer than 1 year, neither disease incidence nor prevalence can be validly estimated by mortality indicators. 23. Which of the following statements pertains to relative survival? a. Refers to survival of first-degree relatives b. Is equal to the case-fatality rate c. Is generally closer to observed survival rates in younger age groups d. Is generally closer to observed survival rates in older age groups e. Provides an estimate of proportionate mortality Rationale: Relative survival is close to observed survival rate when there are few competing causes of death. This occurs primarily in younger age groups who are less likely to experience mortality events compared to older age groups. 24. What was the probability of surviving the second year given survival to the end of the first year? a. The probability of surviving the second year given survival to the end of the year indicates that we are concerned with the survival proportion of those alive at the end of year 1. In this example, we have 950 persons alive at the beginning of year 2 (thus, the end of year 1). Of this group, 30 die by the end of the second year. This gives a survival rate of 920 divided by 950, or 97%. 25. What was the cumulative probability of surviving after only 2 years of follow-up? a. The cumulative survival is the total number of those surviving by the end of the second year divided by all persons who were alive at the beginning of follow-up. In this example, there were 920 survivors among the 1,000 persons who were alive at the beginning of observation. This equals a cumulative survival of 92%. Alternatively, this cumulative survival can be calculated by multiplying the survival rates for each period of interest. In this example 95% survival for year 1 multiplied by 97% survival for year 2 equals a cumulative survival of 92%. 26. An important assumption in this type of analysis is that: a. No change has occurred in the effectiveness of treatment during the 3-year period b. Rationale: An important assumption of survival analysis is that separate strata, in this example, years of follow-up, have similar underlying rates of survival. If some external factor were to differentially influence survival during a portion of the follow-up time then we would not be able to assume a cumulative survival that is consistent during the entire study period. 27. Complete the table. What is the probability that a person enrolled in the study will survive to the end of the third year? a. The answer is 48.6%. b. Completing the table gives the following results for each column: c. Column 5 from top to bottom: 350, 255, 184 d. Column 6 from top to bottom: 0.229, 0.228, 0.185 e. Column 7 from top to bottom: 0.771, 0.772, 0.815 f. Column 8 from top to bottom: 0.771, 0.596, 0.486 Rationale: The cumulative survival at the end of the follow-up period equals the probability of survival during each of the years of follow-up. In this example, multiplying 0.771 by 0.772, then multiplying this product by 0.815 equals the cumulative survival rate of 0.486. 28. Before reporting the results of this survival analysis, the investigators compared baseline characteristics of the 38 people who withdrew from the study before its end to those who had complete follow-up. This was done for which of the following reasons: a. To check whether those remaining in the study represent the total study population b. Rationale: A key assumption for the use of survival analysis is that persons who are lost to follow-up have the same mortality experience as those remaining in the study. The failure to satisfy this assumption introduces a bias in the survival estimates since the observed population has different attributes that are associated with survival compared to the population that is lost to follow-up. 29. Which of the following is a key assumption involved in the use of life-table analysis? a. The risk of disease does not change within each interval over the period of observation b. There are no losses to follow-up in the study population c. The frequency of exposure is similar in treatment & comparison groups d. The disease is common e. The study subjects are representative of the population from which they were drawn Rationale: Life-table analysis depends upon a consistent rate of survival during all periods of the study. Changes in the rate of survival may be due to external influences that are operating at later times on only a portion of the initial population. Since those who have died earlier in the study period will not experience these external influences, the comparison between periods is rendered invalid. 30. Which of the following is a measure of disease prognosis? a. Prevalence b. Median survival time c. Age-adjusted mortality rates d. St&ardized mortality ratio e. Proportionate mortality ratio Rationale: Disease prognosis indicates the likelihood of survival once a disease has become manifest. The median survival time reflects the length of time that the 50th percentile of affected persons has. It differs from the mean survival time in that the mean survival time is an average that may be influenced by extremely low or high survival times. The median survival time consists of an ordering of all survival times with the midpoint of the distribution taken as the duration of survival. 31. In 2003, Sudden Acute Respiratory Syndrome (SARS) appeared in several countries, mainly in Asia. The disease was determined to have been caused by a virus that could be spread from person –to person from the index case occurring in mainl& China. This table reflects the total number of reported cases of SARS & deaths among those cases as best as can be determined. What is the overall case-fatality rate for the worldwide epidemic of SARS? a. 9.5% b. Rationale: This can be found by dividing the total number of deaths due to SARS by the total number of cases. This equals a case-fatality rate of 9.5%. 32. In 2003, Sudden Acute Respiratory Syndrome (SARS) appeared in several countries, mainly in Asia. The disease was determined to have been caused by a virus that could be spread from person –to person from the index case occurring in mainl& China. This table reflects the total number of reported cases of SARS & deaths among those cases as best as can be determined. Based on the table, we can conclude that the case-fatality rate (CFR) in Vietnam: a. Is the same as the case-fatality rate in Singapore b. Is twice as great as the case-fatality rate in Singapore c. Is almost one half that of the case-fatality rate in Singapore d. Cannot be determined because the data are not age-adjusted e. Depends on the number of secondary cases Rationale: The CFR in Vietnam equals 5 divided by 63, or 7.9%, while that of Singapore equals 15%. This is approximately one half the rate. 33. What happened to the case-fatality rate (CFR) following this reclassification? a. It was decreased b. Rationale: The increase in prevalent cases with no change in mortality would decrease the CFR since the numerator, number of deaths due to SARS, would stay the same while the denominator, number of cases, increased. 34. What is the probability of surviving the second year of the study given that a person survived the first year? a. The independent probability of surviving the second year for all persons who survived the first year is found by dividing the number of survivors at the end of the period by the total number present at the beginning of the period. In addition, for those who withdraw during the interval, only 50% of these persons should be counted as being present during the interval. The table should be completed with the following values: b. Column (B) from top to bottom: 248, 124, 55 c. Column (E) from top to bottom: 0.410, 0.470, 0.296 d. Column (F) from top to bottom: 0.590, 0.530, 0.704 e. Column (G) from top to bottom: 0.590, 0.313, 0.220 f. Therefore, the second year survival probability among all those surviving in the study past the first year is 53%. The probability of dying during the second year equals the number of deaths during the interval (55) divided by the total number of persons alive at the start of the interval less one half of those withdrawing from the study (117). Subtracting this value from 100% results in a survival rate of 53% for the interval. 35. For all people in the study, what is the probability of surviving to the end of the second year? a. The cumulative probability of survival through the second year equals the probability of survival for the first year multiplied by the probability for the second year. This equals 59% multiplied by 53%, or 31.3%. 36. What is the probability chance of surviving 3 years after diagnosis? a. The cumulative survival probability for all 3 years equals the product of the independent interval survival probabilities. In this example, 59% multiplied by 53% multiplied by 70.4% gives a cumulative survival probability of 22%. 37. What is the total number of person-years of follow-up for patients in the study assuming a median survival time of one half of the year for all persons dying during an interval & an observation time of one half of the year for all persons withdrawing from the study? a. This calculation involves attributing the correct amounts of person-years to each group during an interval. For the first year of the study, 96 deaths occur. Using the median survival time, we can calculate that these persons contributed 48 person-years of observation. Additionally, 28 persons withdraw from the study. Again, allocating one half of the year to each of these patients results in 14 person-years. Of the remaining 124 persons who survive for the full year, they contribute 124 person-years of observation. The total person-time for the first year of the study is 186 person-years. Continuing with this same approach for years 2 & 3 of the study, we arrive at a total of 321.5 person-years of observed study time. 38. Before reporting the results of this survival analysis, the investigators compared baseline characteristics of the 44 people who withdrew from the study before its end to those who had complete follow-up. This was done: a. To check whether those withdrawing from the study are similar to persons remaining in the study b. Rationale: A key assumption in life table analysis is to insure that the experience of those lost to follow-up, or withdrawals from the study, is the same as those remaining under observation.