Which type of epidemiological study is being described in each of the following scenarios? Classifications of each study are below.

1. A study that examines the death rates from colon cancer in each of the 50 U.S. states in relation to the average percentage of residents in each state undergoing colonoscopy screening

2. A study that compares the prevalence of back pain among current members of the automobile manufacturing union with that of current members of the bakers and confectionary union

3. A study that evaluates the relationship between breast cancer and a woman’s history of breastfeeding. The investigator selects women with breast cancer and an age-matched sample of women who live in the same neighborhoods as the women with breast cancer. Study subjects are interviewed to determine whether they breastfed any of their children.

4. A study that evaluates two treatments for breast cancer. Women with stage 1 breast cancer are randomized to receive either cryotherapy (a new treatment involving extreme cold to kill cancer cells) or traditional lumpectomy. Women are followed for 5 years to determine whether there are any differences in breast cancer recurrence and survival.

5. A study that began in 2010 of the relationship between exposure to chest irradiation and subsequent risk of breast cancer. In this study, women who received radiation therapy for postpartum mastitis (an inflammation of the breast that occurs after giving birth) in the 1950s were compared with women who received a nonradiation therapy for postpartum mastitis in the 1950s. The women were followed for 60 years to determine the incidence rates of breast cancer in each group.

Type of studies

Experimental

Studies preventions and treatments for diseases; investigator actively manipulates which groups receive the agent under study.

Observational

Studies causes, preventions, and treatments for diseases; investigator passively observes as nature takes its course.

Cohort

Typically examines multiple health effects of an exposure; subjects are defined according to their exposure levels and followed for disease occurrence.

Case–control

Typically examines multiple exposures in relation to a disease; subjects are defined as cases and controls, and exposure histories are compared.

Cross-sectional

Typically examines the relationship between exposure and disease prevalence in a defined population at a single point in time

Ecological

Examines the relationship between exposure and disease with population-level rather than individual-level data