Session II. Study Design. Session Overview. Developing and testing hypotheses Study Designs: Selection Implementation Sampling. Learning Objectives. Understand the differences in methodology between various study designs
A study considers two main factors:
exposure and outcome
Two types used in outbreak investigations
In epidemiology, “Any designated group of individuals who are followed or traced over a period of time.”
- Last, JM. A Dictionary of Epidemiology, 3rd ed. New York: Oxford University Press, 1995
A cohort study analyzes an exposure / disease relationship within the entire cohort.
MMWR: December 13, 2002 / 51(49);1112-1115
Iwamoto M, Hlady G, Jeter M et al. Shigellosis among Swimmers in a Freshwater Lake-Georgia, 2003. Presented at the 53rd Annual Epidemic Intelligence Service Conference. Atlanta, GA. April, 2004.
Begier EM, Barrett FK, Mshar PA et al. Body Shaving, Whirlpools, and Football: An Out break of Methicillin-Resistant Staphylococcus aureus Cutaneous Infections in a College Football Team-Connecticut, 2003. Presented at the 53rd Annual Epidemic Intelligence Service Conference. Atlanta, GA. April, 2004.
Steps in a Case-Control Study:
Step 1 - Identify source population
Step 2 – Establish a case definition
and select cases
Step 3 – Select controls
Step 3 – Select controls (cont’d.)
Sampling is the systematic selection of a portion of the larger source population. A sample should be representative of the larger source population.
Because it is more efficient – saves time and money!
Is the purpose of the study to determine the source of the outbreak?
Is the purpose of the study to determine the number of persons who become sick over a specific period of time?
Types of sampling
Simple random sample (SRS)
Randomly select persons to participate in study. There are many variations of SRS.
Choose those individuals who are easily accessible.
Problems with convenience sampling