PubH 6003: Principles and Practice of Epidemiology. Case Study # 1: MEASURES OF DISEASE FREQUENCY

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Case Study # 1: MEASURES OF DISEASE FREQUENCY PubH 6003: Principles and Practice of Epidemiology 2 There are a total of 68 points for this case study. Please read each problem carefully and sho ... w all your work. Please round each of your answers to the nearest 10th decimal point (e.g., 3.258 = 3.3). ************************************************************************ Outbreaks of Gastroenteritis on Cruise Ships Recently, a report in the Centers for Disease Control (CDC) publication Morbidity and Mortality Weekly described a string of individual cases of Norwalk virus gastroenteritis occurring on cruise ships. From this report, the editor went on to form a number of hypotheses as to why there has been and this rather unusual increase in reported gastroenteritis outbreaks on cruise ships in 2002. The cruise ship owners contacted the CDC to conduct an in-depth analysis of the possible modes of transmission of the Norwalk virus in the cruise ship environment. CDC investigators interviewed all of the passengers on the last affected cruise (N=3,000) and obtained information on the passenger’s recreational activities. They found the following results: 1,000 passengers had gone swimming in the upper deck pool and 2,000 passengers had never gone swimming in the upper deck pool. 100 of the passengers who swam in the upper deck pool and 100 of the passengers who did not swim in this pool developed Norwalk virus gastroenteritis during the cruise. FYI: The cruise lasted one week. 1. Set up the two by two table for these data (4 pts.) 2. Compare the risk of becoming ill for those who never swam in the upperdeck pool with the risk for those swimming in the upper deck pool (2 pt.). 3 3. How many times more likely are you to become ill if you swam in the pool compared to passengers who did not? ( 1 pt.). 4. How much greater is your risk if you swam in the pool than the risk for those who did not? (1 pt.). . 5. What measure of disease frequency are you using to calculate these comparisons (1 pt.)? 6. Which comparison should be used to answer the question, “How many additional cases of Norwalk virus gastroenteritis among all cruise passengers (N=3,000) was associated with swimming in the upper deck pool?” (1 pt.) When the cruise ship owners examined the findings of the CDC investigation, they stated that the crude results (calculated above) were invalid because of the age differences between the people who swam in the upper deck pool and those who did not. Examine the following table and state whether or not you agree with the cruise ship owners. 7. What is it about the above data would back-up or refute the CDC assertion. (1 pt.). 4 8. Regardless of whether you agree with the cruise ship owners, explain in at least 3 sentences the method that epidemiologists use to account for age differences in populations. Be sure to mention what additional data would be needed to perform this procedure (2 pts.). Tuberculosis in a Housing Community In January 2010 you began a one-year study of tuberculosis (TB) in a subsidized housing community in the Lower East Side of New York City. You enrolled 500 residents in your study and checked on their TB status on a monthly basis. At the start of your study on January 1st, you screened all 500 residents. Upon screening, you found that 20 of the healthy residents were immigrants who were vaccinated for TB and so were not at risk. Another 30 residents already had existing cases of TB on January 1st. On February 1st, 5 residents developed TB. On April 1st, 5 more residents developed TB. On June 1st, 10 healthy residents moved away from New York City were lost to follow-up. On July 1st, 10 of the residents who had existing TB on January 1st died from their disease. The study ended on December 31, 2010. Assume that once a person gets TB, they have it for the duration of the study, and assume that all remaining residents stayed healthy and were not lost to follow-up. 9. Is the subsidized housing community in the Lower East Side of New York City a dynamic or fixed population? Briefly explain the rationale for your answer (1 pt.). 10. What was the prevalence of TB in the screened community on January 1st (1 pt.) 11. What was the prevalence of TB on June 30th (1 pt.)? 12. Given these data, what is the best estimate of cumulative incidence of TB over the year (1 pt.)? 13. What is problematic about this measurement? (1pt) 5 14. Suppose that you wanted to calculate the incidence rate of TB in the study population. Calculate the amount of person-time that would go in the denominator of this incidence rate. Be sure to show your work (1 pt.). 15. Compare the cumulative incidence to the incidence rate calculated using person years. Comment on the difference and what it means? (1 pt) 16. What was the case-fatality rate among residents with TB over the course of the year (1 pt.)? 17. Is this a good measure of the case fatality for TB? (1 pt) Measures of Disease Frequency and Association Consider a group of 10,000 newborn infants. 600 infants were born with serious birth defects and 80 of these 600 died during the first year of life. 450 of the 9,400 remaining infants without any birth defects also died during the first year of life. 18. Calculate the prevalence of serious defects in this population at the time of birth (1 pt.). 19. Calculate the overall cumulative incidence of mortality in this population (1 pt.). INFANT DIED FROM DEFECT = 80 6 20. How many times greater is the risk of death in infants with serious birth defects than in children without serious birth defects? (1 pt.). 21. How many deaths would be eliminated if none of the infants had serious birth defects? (1 pt.). How does each of the following conditions influence the prevalence of a disease in a population? For each scenario, assume that no other changes occur. Your choices are: increases prevalence, decreases prevalence, or has no effect on prevalence. 22. A treatment is developed that prolongs the life of people suffering from the disease (1 pt.) 23. A new measure is developed that prevents new cases of disease from occurring (1 pt.) 24. There is immigration of a large number of healthy people into the population (1 pt.) 7 In January, 1999 forty heterosexual hemophiliac patients (all males) who periodically received intravenous infusion of blood products to control their hemophilia were asked to participate in a 3 year prospective study to determine their risk of three adverse outcomes: a) the rate of HIV (human immunodeficiency virus) seropositivity, b) the rate of developing clinical signs of AIDS, and c) death from AIDS. The men were to undergo an interview, physical examination, and blood testing every 6 months for 3 years. Among the 40 subjects there were 30 who were seronegative and healthy for the entire duration of the study, and all of these were followed for the entire 3 years. During the initial screening, 2 of the men were found to already be HIV+, although none of them had clinical signs of AIDS. The table below describes the 10 subjects who either developed AIDS, or became lost to follow-up, or already had AIDS at the start of the study. Legend: HIV+ = found to be HIV+ at the very beginning of the interval observation period ? = lost to follow-up 25. From the information above, what was the prevalence of seropositivity (HIV+) in Jan. 2000 (1 pt.)? 26. Should the cumulative incidence of seropositivity (HIV+) during the 3 year study be calculated (1 pt.)? 27. What was the incidence rate of seropositivity (HIV+) during the study? (Show your work.) (1 pt.) A group of 100 healthy women was followed prospectively for 10 years. All subjects entered the study on January 1, 1990 and all women were followed until December 31, 1999. None were lost to follow-up. During this period, 5 subjects were diagnosed with breast cancer, but they all survived to the end of the study. The time at which these 5 subjects developed cancer is shown in this table. Assume that each diagnosis occurred exactly half way through the year. Subject 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 1 cancer 2 cancer 3 cancer 4 cancer 5 cancer 28. What was the cumulative incidence of breast cancer (1 pt.)? 9 29. What was the incidence rate of breast cancer (1 pt.)? 30. What was the prevalence of breast cancer “survivors” among the entire population on December 31, 1999 (1 pt.)? Suppose that you are following a group of children for the development of asthma over a one-year period. You identify 100 children on January 1st, 2005, screen them for asthma, and set up a monitoring program to check on their status on a monthly basis. Five children are considered prevalent cases because they were diagnosed with asthma before January 1st. Ten children develop asthma on March 1st and another 10 children develop asthma on July 1st. Another 10 children who remain healthy were followed for six months and then were lost to follow-up. All of the remaining children did not develop asthma and were not lost to follow-up. Follow-up ended on December 31st., 2005. 31. How many person-months of observation were accrued by this population? (2 pts) The following table lists the age-specific number of deaths and populations for two communities, A and B. 10 Place the answers to the questions 32 – 34 in the appropriate cells in the above two tables: 32. Calculate the crude mortality rates for populations A and B. Which population is healthier? (2 pts.) 33. Calculate the age-specific mortality rates for populations A and B. (5 pts.) 34. Calculate the expected number of deaths for Populations A and B using the agespecific U S population as the standard population. (5 pts.) 35. What are the age-standardized mortality rates for populations A and B? What method of rate standardization did you use to calculate these adjusted rates? (3 pts.) The following two tables presents data on mortality in two factories, A and B. Use these data to answer Question 36-38. DATA ON FACTORY A: DATA ON FACTORY B: Place the answers to the following questions in the appropriate cells in the above two tables: 36. Calculate the expected number of deaths for each factory, A and B. (3 pts.) 37. Calculate the Standardized Mortality Ratio (SMR) for each factory, A and B. (2 pts.) 38. Interpret the meaning of each SMR you calculated. Why are the SMR’s different between Factories A and B? What method of rate adjustment or standardization did you use to calculate the SMRs? (3 pts.) The following table lists selected causes of death by gender, for persons age 15-24, for the State of New Hampshire for the years 2002-2006. Use these data to answer the following questions: Table 1: Selected Causes of Death*, New Hampshire, 2002-2006, Age 15-24, Males and Females Males Females 12 * Deaths are 5-year totals 5-Year Population Totals, Ages 15-24 Males 765,720 Females 792,650 Total 1,559,370 39. Calculate the age, sex, and cause-specific mortality rates for each cause of death for both males and females. Record your answers in the appropriate cells in the Table (4 pts.) 40. Is the leading cause of death the same for males and females? (1 pt.) 41. Is the suicide rate greater for males or for females? What is the ratio of the suicide rates for males to the suicide rate for females? How would you express this ratio in common terms? (3 pts) [Show More]

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