Homework1 - CE/CH/ENV-54 Homework 1 Spring 2008 1 Suppose...

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2/14/08 CE/CH/ENV-54: Homework # 1, Spring 2008. 1. Suppose you are investigating an outbreak of a disease in a small area (Region A) of a country. Your team does a random survey of 1,000 people who live in Region A and find that there are 193 cases. What measure(s) of disease occurrence can be calculated here? Calculate and interpret the measure(s). Also calculate and interpret a 95%CI for any measure you calculate. Prevalence: 193/1,000= 0.193. ~19% of the people have the disease. 95% CI = 0.193 + and - 1.96√[0.193(1 – 0.193) / 1000] 95% CI = [0.17, 0.22] We are 95% confident that the true prevalence of the disease among people in Region A falls between 17% and 22%, with the best estimate at 19% 2. Your boss at the Centers for Disease Control has you go back to the same area six months later. Your team does another random survey of 1,000 people (all are different from the subjects in #1) and finds 130 cases. What measure(s) of disease occurrence can you calculate? Calculate and interpret the measure(s) and 95%CI(s). Your boss wants to know if you can tell him if the current situation is better, about the same, or getting worse compared to the first survey six months ago. How would you answer? Prevalence: 130/1,000= 0.13. 13% of the people have the disease 95% CI = 0.13 + and - 1.96√[0.13(1 – 0.13) / 1000] 95% CI = [0.11, 0.15] We are 95% confident that the true prevalence of the disease among people in Region A falls between 11% and 15%, with the best estimate at 13% The results of the surveys can not be compared; even though we have prevalence calculated at two separate times, risk can not be calculated because the point estimates are from different samples. 3. Your boss wants more information. You’re sent back to Region A to enroll 2,000 people in a study. All subjects are free of the disease at the time of enrollment. After three months, 139 of the people have contracted the disease of interest. What measure(s) of disease occurrence can be calculated? Calculate the measure(s) and 95%CI(s). Interpret.
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