Epidemiology Homework

Epidemiology Homework

Problem 1 (10 points; 3 points each +1)

A population of 1,000 people is monitored for a year for the development of measles. No one has measles at the start of the investigation. Thirty people develop

measles on June 30 and twenty people develop measles on September 30. Eight people are lost to follow-up on March 31 and twenty-four people are lost to follow-up on

November 30. None of those lost to follow-up had developed measles prior to becoming lost. Assume that you can only get measles once.

A. What is the cumulative incidence of measles in this population?

B. What is the incidence rate of measles?

C. What is the prevalence of measles on July 1?

Problem 2 (12 points; 3 points each)

Suppose that a study of oral contraceptive (OC) use and development of bacteriuria was conducted among 2,390 women, all of whom were initially free from bacteriuria.

At the start of the study, the women were surveyed to determine whether or not they currently used OCs. 482 were using OCs and 1908 were not. By the end of the

follow-up period, a total of 27 of the OC users and 77 of the non-users had developed bacteriuria.

A. Is this a cohort study or an experimental study?

B. Set up the two-by-two table for these data.

C. Calculate the risk ratio describing the strength of the relationship between OC use and bacteriuria.

D. Based on your answer above, do you think that OC users are at higher risk of developing bacteriuria than non-users?

Problem 3 (20 points; A=8 points, B=3×2 calculations and 3×2 interpretation)

Suppose that a cohort study of body mass index and coronary heart disease (CHD) was undertaken. Participants were classified as having either a high or normal body

mass index. Because age is associated with both body mass index and risk of CHD, age was considered a potential confounder and the age of each participant was

recorded. The following data were gathered from the study participants:

Normal Body Mass Index

High Body Mass Index


Young Old


Young Old

Developed CHD



Developed CHD



Did Not Develop CHD



Did Not Develop CHD









A. Set up the two by two to evaluate the association between body mass index and CHD and calculate the appropriate measure of association. (4 points table, 4 points

RR crude)

B. Set up the two by two tables to evaluate the relationship between body mass index and CHD stratified by age. Calculate and interpret the measures of association for

each of the two age groups (8 points each for RR young, RR old, and interpretation)

PROBLEM 4 (16 points total; 2 point each)

Which type of measure of disease frequency best describes each of the following scenarios?

A. Percentage of students enrolled in a college who developed influenza during the spring semester of 2012.

B. Percentage of students enrolled in an epidemiology class who had sore throats on the first day of class.

C. Percent of breast cancer patients who underwent mastectomy during 2012.

D. Percent of men found to have high blood pressure at their yearly physical.

E. Number of newly-diagnosed cases of AIDS in a year per 100,000 persons.

F. Percent of infants born with spina bifida out of 1,000 liveborn infants.

G. Percentage of children developing measles in a study in which children were followed for an average of 2.5 years.

H. Percent of drivers found to be legally drunk at the time of their car accident.

PROBLEM 5 (10 points; 2.5 points each)

Consider a group of 2,000 newborn infants. 200 infants were born with heart defects and 40 of these 200 died during the first year of life. 180 of the 1800 remaining

infants without heart defects also died during the first year of life.

1. Calculate the prevalence of serious defects in this population at the time of birth.

2. Calculate the overall cumulative incidence of mortality in this population.

3. Calculate the cumulative incidence difference in mortality between infants born with serious birth defects and without.

4. State in words your interpretation of the cumulative incidence difference calculated in part c.

PROBLEM 6 (10 points; 3 points each +1 point)

A group of 100 healthy women was followed prospectively for 10 years. All subjects entered the study on January 1, 2000 and all women were followed until December 31,

2009. 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.






















What was the cumulative incidence of breast cancer?

What was the incidence rate of breast cancer?

What was the prevalence of breast cancer “survivors” on December 31, 2009?

PROBLEM 7 (12 points; 3 points each)

A study followed 900,000 US adults from 1992 to 2008. At baseline, all participants were screened and determined to be cancer free and their body mass index (BMI) was

calculated. Body mass index is a measure of obesity that is calculated using a person’s height and weight. Subjects were separated into the following groups according

to their BMI: (a) normal weight, (b) slightly overweight, (c) moderately overweight and (d) greatly overweight. 57,145 deaths from cancer occurred in the population

during the follow-up period.
1. What type of study is this?

2. Use the data given above to calculate the cumulative incidence of deaths from cancer among the study population over the follow-up period.

3. What additional information would need to be provided for you to be able to calculate the incidence rate of cancer deaths?

4. The following results were seen for men and women when the heaviest members of the cohort were compared to those with normal weight:

Men: Risk ratio of cancer death = 1.5

Women: Risk ratio of cancer death = 1.6

State in words your interpretation of the risk ratio given for the men and for women.

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