NR 503 Week 8 Final Exam;Questions and answers chamberlain college of nursing, Exams of Nursing

Download NR 503 Week 8 Final Exam;Questions and answers chamberlain college of nursing and more Exams Nursing in PDF only on Docsity! NR 503 Week 8 Final Exam;Questions and answers chamberlain college of nursing Chapters 2-4 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 Rationale: The incubation period is defined as the interval from receipt of infection to the time of onset of clinical illness. Accordingly, individuals may transmit infectious agents during the incubation period as they show no signs of disease that would enable the isolation of sick individuals by 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 Rationale: The disease is spread by contact with an infected individual who can transmit the agent (VZV) to immunologically naive persons during the incubation period & for several days after onset of clinical illness. Since the boy started school 14 days after showing signs consistent with chicken pox, it is most likely that he was no longer infectious. 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 Rationale: Single-exposure, common-vehicle outbreaks involve a sudden, rapid increase in cases of disease that are limited to persons who share a common exposure. Additionally, few secondary cases develop among persons exposed to primary cases. A histogram of the outbreak can plot the number of cases by time of disease onset. In single-exposure, common-vehicle outbreaks, a log transformation of the time of disease onset will often take on the characteristic shape of a normal distribution (i.e., a bell curve) with the median incubation time found at the peak of the curve. 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 Rationale: The attack rate in this example is defined as the number of persons who develop diarrhea divided by the total number of people at risk. In this example, the at-risk group is those who have eaten both ice cream & pizza. Of these 52 persons, 39 developed diarrhea. 5. What is the overall attack rate in persons who did not eat ice cream? a. 30% b. 33% c. 35% d. 44% e. Rational e: 58% The attack rate is the number of persons with diarrhea (14 + 9) divided by the total number of persons who did not eat ice cream (40 + 30). 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 Among persons eating ice cream, over 70% developed diarrhea 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. Rationale: Women who have had hysterectomies (i.e., removal of the uterus) are no longer at risk for uterine cancer. For women older than 50 years of age, this would increase the age-specific incidence rate as there would be the same number of uterine cancers occurring among fewer women at risk. Further, rates may decrease among younger women who have had hysterectomies as they are no longer at risk for uterine cancer & thus may decrease the number of potential cases occurring in their age group over time. 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 Rationale: The survey reports the disease status of a population at a specific point in time. In this case, a r&om sample of adult males in 2005 provides a reliable estimate of the prevalence of hypertension. Since there is no information on duration of hypertension in these men, incidence cannot be calculated. Therefore, the researchers are not able to make a statement concerning risk of hypertension in the population. 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. Rationale: Prevalence & incidence are related by the duration of disease. If incidence is increasing over time, then duration of illness has to decrease in order to keep the prevalence rate constant. This may occur through better treatments to cure disease or through higher case-fatality rates as a disease becomes more lethal. Since incidence is increasing over time, it is evident that risk is also increasing & that prevention efforts are not successful. A prevalence survey conducted from January 1 through December 31, 2003 identified 580 new cases of tuberculosis in a city of 2 million persons. The incidence rate of tuberculosis in this population has historically been 1 per 4,000 persons each year. 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 Active surveillance entails a concerted effort to collect information about disease occurrence. It typically involves dedicated staff members who have been specifically directed to contact physicians & hospitals in order to collect reports of disease cases in a specified population. This activity requires a large amount of staff & resources in order to accomplish its goals. 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 Rationale: With the implementation of the lab test, the increase in early detection of cases will increase incidence, duration, & prevalence; however, since the prognosis is still the same, at least 80% of patients will die during the year 2000. This should result in a similar mortality rate as the previous year given no change in transmission, prevention, or medical care of the disease. 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? Answer: The answer is 41.7 per 1,000 persons. The rate is calculated by dividing 100,000 deaths by the population of 2,400,000 persons. To express as a rate per 1,000 persons, the rate is multiplied by 1,000. 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? Answer: The answer is 81. The ratio is calculated by dividing 192 observed deaths by the 238 expected deaths for this age group. To express it as an SMR, the ratio is often multiplied by 100. 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. 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). 16.16. 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 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. 31.31. 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. 34.34. 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.