Norwalk virus gastroenteritis among passengers on cruise ships

Question 1 
Recently, a report was published describing a string of cases of Norwalk virus gastroenteritis among passengers on cruise ships. From this report, an epidemiologist went on to form a number of hypotheses as to why there had been this rather unusual increase in reported gastroenteritis outbreaks on cruise ships in 2012. The cruise ship owners contacted the Centers of Disease control and Prevention (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.
· Set up the 2×2 table for these data.
· Calculate the risk ratio of gastroenteritis associated with swimming in the upper deck pool.
· State in words your interpretation of the above risk ratio
· Calculate the risk difference in the above example
· State in words your interpretation of the above risk difference
Question 2 
For this problem, note the following chart:Age Group
(in years)
% of Population in Age Group
Influenza Rate per 1,000   person-years

There are 10,000 individuals in City A, which is located in Massachusetts. Eight young individuals and 420 old individuals develop the flu over the course of a year.
· Use these data to calculate the crude influenza rate per 1,000 individuals per years in City A.
· What is the crude rate of influenza in City B?
· What is the crude rate of influenza in City C?
· Calculate an age-adjusted influenza rate for each of the cities. Use the age distribution for the State of Massachusetts (shown in the table) as the standard.

Question 3 
Recently, Australian researchers conducted a study of the relationship between optimism and colon cancer survival. Their hypothesis was that colon cancer patients who had a positive outlook on life would have a lower five-year cumulative incidence of mortality. The study included 100 recently diagnosed colon cancer patients who underwent psychological testing and were found to have a optimistic outlook on life and 100 recently diagnosed colon cancer patients who underwent the same psychological tests and were found to have a pessimistic outlook on life. By the end of five years of follow-up, 50 of the 100 patients with the optimistic outlook and 75 of the 100 patients with the pessimistic outlook had died from colon cancer.
· Set up and fill in the two by two table using these data.
· What is the prevalence of colon cancer in the study population?
· Compare the cumulative incidence of mortality in the optimistic group to the cumulative incidence of mortality in the pessimistic group using a ratio measure of association.
· State in words your interpretation of the result you found in part c.