Problem setEpidemiological measures

Epidemiology: Module 3 Problem Set

Equal case counts in unequal groups, which is what makes the risk ratio worth calculating. Then the harder question: whether the pool is a plausible route for this organism at all.

Editorial process

Last reviewed · August 22, 2026

Written by

Maren Caldwell

MSN, RN, CNE

Medical-surgical nursing, pharmacology and NCLEX preparation

Reviewed by

Dr. Tessa Redmond

DNP, RN, CNE

Evidence-based practice and clinical education

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Interpret the task
Assignment instructions
Deliverables
Proposed outline
Research plan
Reference shortlist
Final review

Equal cases, unequal groups, so read the risks not the counts

Build the table before doing any arithmetic, and lay it out the conventional way with exposure down the side and outcome across the top, so exposed cases, exposed non-cases, unexposed cases and unexposed non-cases each have a cell. The numbers are chosen to catch anyone who reads case counts instead of risks: a hundred cases occurred in each group, which looks like no difference, but the exposed group has a thousand people and the unexposed group has two thousand. So the risk among swimmers is a hundred in a thousand and the risk among non-swimmers is a hundred in two thousand, and the ratio of those is two. Write both risks out before dividing them: the arithmetic is trivial, but showing the two denominators is what demonstrates you understood why the equal case counts were a trap rather than a coincidence, which is the whole point of the question.

Then write the interpretation as a sentence rather than a number, because the question asks for it in words and this is where marks are lost. A risk ratio of two means passengers who swam in the upper deck pool had twice the risk of developing gastroenteritis during the cruise compared with those who did not, and the phrase compared with is what a bare number leaves out. The risk difference is the absolute measure and answers a different question: how much of the risk is additional in the exposed group, which here is five per hundred, and it is the measure that matters for deciding whether closing the pool would be worth it. Say what each measure is for rather than reporting both as though they were interchangeable. Finally, an association is not a transmission route. Norovirus spreads person to person and through food and surfaces, and chlorinated pool water is a poor medium, so swimming may be a marker.

Likely learning objectives

Construct a 2x2 table before calculating any measure.
Distinguish case counts from risks when group sizes differ.
Interpret a risk ratio in words with an explicit comparison group.
Distinguish relative from absolute measures and what each decides.

Assignment instructions

Read the full question

Review every instruction before using the planning guidance that follows.

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-year 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. Epidemiology: Module 3 Problem Set

Turn the brief into deliverables

01A correctly laid out 2x2 table with all four cells.
02Risk in each exposure group, shown as a calculation.
03The risk ratio with working.
04An interpretation sentence naming the comparison group.
05The risk difference and what it is used for.
06A note on plausibility and possible confounding.