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EPIDEMIOLOGY 200A
Drs. Olsen & Arah Fall 2009 EXERCISE 2: DISEASE OCCURRENCE Problem 1. For a population of 10,000 in steady-state you have been told the following about a certain disease and an exposure of interest: 40% of your population reports exposure to the risk factor in a year One-year risk of disease among the exposed is estimated to be 0. One-year risk of disease among the unexposed is estimated to be 0. Fill in the following 2 x 2 table based on the one-year information given above.
Disease No Disease Total
Exposure
No Exposure
Total 10,
How many cases of this disease do you expect to see in one year? What is the estimated prevalence of this disease among the population? What is the risk ratio? Interpret your result in words. What is the risk difference? Interpret your result in words.
Among the exposed, what is the proportion of disease that can be attributed to the exposure? You work for the mayor of this community and she says to you, “What proportion of disease could we eliminate from our population if we were to eliminate this exposure?” What will you tell her? Problem 2. What is the difference between incidence and prevalence? Problem 3. How are prevalence and incidence related? Problem 4. For each of the following scenarios, determine whether it more closely resembles incidence or prevalence: Number of campers who developed gastroenteritis within 24 hours after eating potato salad at the dining hall Number of persons who reported having hypertension as part of a national survey. Occurrence of Acute Myocardial Infarction in participants during the first 10 years of follow up in Framingham Heart Study.
Problem 8 A closed cohort of 100 subjects was examined for a disease at baseline and 2 years later. 20 subjects had the disease at baseline. Additionally, 30 new cases had the disease at follow-up; however, 18 existing cases (among the 20 cases at baseline) were accidentally counted again as part of the 30 cases at follow-up. The actual two-year risk of disease is: a. 0% b. 12% c. 13% d. 15% e. 20% Problem 9 20 new cases of STDs occurred during a 5 year follow-up period in a closed cohort of 100 women aged 20-30 years old, who were screened as non-cases of STDs at the beginning of the follow-up. The 5 years risk of STDs in this cohort is: a. 10% b. 18% c. 20% d. 24% e. 27% Problem 10 During the period 1930-1970, the annual number of deaths due to cancer in the United States increased from 118,000 to 331,000. One explanation for this steep increase could be an increased exposure of the population to carcinogenic substances. What are some other possible explanations? Problem 11 A study published in 1990 (Am J Pub Health 80:p. 209-210) investigated the occurrence of HIV infection among prisoners in Nevada. Of 1105 prison inmates who were tested for HIV upon admission to the prison system, 36 were found to be infected. All uninfected prisoners were followed for a total of 1207 person-years and retested for HIV upon release from prison. Two of the uninfected inmates demonstrated evidence of new HIV infection. Assuming that the two prisoners were infected during their time in prison: a. Based on the above information, calculate the incidence rate of HIV infection among prisoners in the Nevada prisons. b. Express the incidence rate calculated in part a in terms of cases per 1000 person-years.
c. Calculate the prevalence of HIV infection among the incoming prisoners in the Nevada prisoners at the start of the study. d. What is the size of the population at risk for HIV at the beginning of the study? Problem 12 You are interested in determining the incidence, mortality, and case-fatality due to AIDS in Los Angeles County. You assemble a group of 110 men, who are HIV positive, of whom 100 are at risk of developing AIDS (the other 10 already met the case definition of AIDS due to their low CD4 count and high viral load), and follow them (all 110 of them) for a period of 10 years. During the 10 year follow-up period, 5 of them die of AIDS, and 30 of them meet the case definition of AIDS. Among the 20 of whom were newly diagnosed with AIDS, 4 die within 1 year after infection. Assume that the subjects either died or recovered within 1 year of meeting the AIDS definition, and that the outcome was observed for all incident cases of AIDS. Assume that there are no losses to follow-up and there are no competing risks. a. Estimate the 10 year risk of developing AIDS in this group of subjects. b. During the 10 year follow-up period, 5 of them die of AIDS. Estimate the 10 year mortality risk in the base population. c. Estimate the 1 year fatality risk of dying from AIDS in this group of subjects.
g. What is the total number of person-months at risk during 1996? h. What is the incidence rate (or incidence density) of the disease during 1996? i. Convert the incidence density estimate from question h to the # of cases per 100 person-years. Problem 14 The table below, taken from a paper by Warshauer and Monk ( Am J Public Health 1978; 68:383-388), gives the number of suicides in East Harlem and the South Bronx between 1968 and 1970 (36 months) among white females, by age in 1970. Assume that the population of white females in this area is stable. Age in Size of 1970 Number of 1970 Population Suicides 18-24 34,714 26 25-34 42,489 23 35-54 59,762 31 55-84 63,306 39 Total 200,271 119
a. Estimate the average suicide rate for each age interval and describe the observed age trend– i.e., how the estimated suicide rate varies with age. b. Estimate the following risks of committing suicide (using the exponential formula): i. The 1-year risk for a 20-year-old white woman, ii. The 10-year risk for a 20-year-old white woman, iii. The 67-year risk for an 18-year-old white woman. c. What additional assumptions are needed to estimate the risks in part b?
Subject Months before improvement Subject Months before improvement Subject Months before improvement 1 4 8 18 15 8 2 14 9 13 16 22 3 5 10 8 17 15 4 9 11 13 18 11 5 13 12 19 19 14 6 10 13 15 20 16 7 6 14 17 a. Estimate the average improvement rate for all TD cases during the entire follow-up period. b. Assuming the improvement rate remains constant, calculate the “risk” of improving, by each time interval after discontinuation (make sure you choose appropriate interval sizes!).
c. Assess whether the improvement rate changed during the two-year follow-up period (as assumed in part b) by comparing the average improvement rate in the first 12 months after discontinuation with the average rate in the second 12 months after discontinuation. Problem 16: Vinyl Chloride – Revisited The following table is from your lecture notes after Duck et al. Lancet 1973, Dec13: 1197-1199. Duration of work O E SMR N Man years at risk <10 years 10-14 years 15+ years
How many man-years at risk should there be in each stratum? Show your work. <10 years: 10-14 years: 15+ years:
