Study Types

Question 1

Descriptive Epidemiology

Answer 1

Defines frequency and distribution of diseases and other health related events; scope and impact of disease; help formulate hypotheses about cause of disease

Question 2

Analytic epidemiology

Answer 2

Analyze determinants of health problems- how? Why?; explores etiologic (causal) relationships between exposures and health outcomes; quantify strength of relationship between exposure and risk of disease

Question 3

Case reports

Answer 3

Accounts of a single occurrence of an unusual health related event; detailed description of the symptoms, signs, diagnosis, and treatment of an individual

Strength: call attention to possible emergence of new diseases/epidemics

Limitations: no way to obtain evidence of causes; no comparison group of those who were not exposed

Question 4

Case series

Answer 4

Describes a handful of patients with an usual disease or health related incidence. Describes common features such as the characteristics of affected patients, or facility where treatment is as provided

Strength: call attention to possible emergence of new diseases/epidemics

Limitations: no way to obtain evidence of causes; no comparison group of those who were not exposed

Question 5

Ecological studies

Answer 5

Studies that use aggregate data on populations rather then on individuals; units of analysis are on the group level rather than individuals

Strength: easy and inexpensive; often rely on pre-existing data

Limitations: cannot examine causality; ecological fallacy

Question 6

Cross sectional studies

Answer 6

Data collected at the individual level; unit of analysis is the person; both exposure and outcome are determined simultaneously; can be used to obtain prevalence estimates; 2x2 tables

Strengths: data available at individual level, can construct subgroups for analyses based on export or health outcomes; generalizable if data from representative samples

Limitations: may under represent diseases with short duration; no objective measure of the temporal ordering of exposure and disease (cannot examine causality)

Question 7

Case control studies

Answer 7

Groups of individuals selected on basis of disease; with disease = cases; without disease = controls; data on potential exposures for risk of disease are collected from both cases and controls (these are always retrospective); cases and controls are compared with respect to history of exposure to potential risk factors

Strengths: relatively fast and inexpensive; optimal for rare disease; good design for diseases with long delay between exposure and disease; useful when underlying population is dynamic

Limitations: inefficient for rare exposures; temporal relationship between exposure and disease may be unclear; recall bias; cannot measure disease frequency since individuals with disease are preselected by the investigator

Question 8

Cohort study

Answer 8

Cohort members are selected based on their exposure, or based on their membership in a group and exposure is assessed at baseline; exposure status is not manipulated; cohort members are followed over time to assess onset or diagnosis of disease

Strengths: can directly measure incidence of outcomes; temporality of exposure and outcomes is clear; good for common outcomes that occur within a short time frame; useful for rare exposures

Limitations: need large samples; inefficient for rare or very delayed outcomes; expensive and labor intensive; likely to lose cohort members to follow up

Question 9

Prospective

Answer 9

Straightforward; we enroll people without disease, ask about exposure, as follow them in time until they develop disease or the study ends

Question 10

Retrospective

Answer 10

Reconstruct a cohort of people who are disease free at a time in the past, collect past exposures, and reconstruct their follow up for outcome of interest

Even though people in cohort may have developed disease before the investigator began the particular study, the investigator is still blind to their ultimate disease status at follow up when conducting the historical cohort

Question 11

Randomized control trial

Answer 11

A type of experiment in which individuals, groups, or communities are randomly allocated to receive, or not receive an intervention

Strengths: protect against confounding, protest against bias, provides best evidence for causality

Limitations: very expensive, not suitable for rare outcomes, only as strong as protocol