An epidemiologic hypothesis is a testable statement of a putative relationship between exposure and disease. The hypothesis should be:
- Testable or resolvable
- State the relationship between exposure and disease
- Limited in scope
- Not inconsistent with known facts
- Supported by literature, theory, references
Hierarchy of Epidemiologic Study Designs in the Demonstration of Causality/Prevention
The design of a study contributes to the strength of its findings. Below are the types of studies, in order of increased strength for testing the relevant hypothesis. We will study some of these designs further later in this course.
- Case Study (describing one person with the condition, a case)
- Case Series (series of cases)
- Ecological Study (analysis of group statistics..for example, comparing rates of disease between two countries)
- Cross-Sectional Study (assessing individuals at one time, such as a survey)
- Case-Control Study (studying those with the condition vs. those without)
- Cohort Study (following subjects over time to study the initiation and progression of a condition)
Often, it is not feasible to conduct a study where we collect data from all affected individuals. Thus, we need to select a subset of those individuals for our study. The following defines populations and samples.
- Target Population
- Population to which inferences from the study are to be made.
A target population may be defined by geography, demography, health status, or some other factor.
- Study Population
- Population from which study subjects are selected
The sampling frame is the actual list that will be used to select the sample (ex. list of hospital admissions, household addresses, people with a certain disease or outcome)
Careful consideration should go into identifying the sampling frame for a study. If one cannot be created that mostly covers the population, bias can occur.
- Subjects that provide data to the study
Data from the study participants are used to make estimates and draw conclusions about the population
Obviously, the method for selecting the sample can greatly influence the study results. In Lesson 2, we will learn more about methods to select samples.