Observational studies

Observational studies can be classified by their design and this is based on:

1. Time- (past, current or future)
2. When the exposure is measured
3. When the disease occurrence is ascertained

Let’s look at some examples:

• Cohort study
  This study begins with exposed and non-exposed groups and then studies the occurrence of the disease.
• Case-control study
  This study begins with cases (diseased subjects) and the controls (non-diseased subjects) and then investigates their exposure status.
• Cross-sectional study
  This is where the exposure and disease are measured concurrently.
• Ecological studies
  are widely used in preliminary descriptive epidemiology, particularly in describing differences between different geographical areas or regions and in different ethnic and cultural groups. In an ecological study the outcome event (usually the disease under study) is not linked to individual subjects but to a group of subjects leading to the possibility of the ecological fallacy (an observed relation in grouped data which is not supported by subsequent data at the individual level).