The exposure is the independent factor of interest in your study — what you think causes the outcome.
The most basic form of the hypothesis is: X causes Y, where X is the exposure.
Example: Smoking causes cancer. Smoking is the exposure.
(exposures are in italics)
Your exposure should not be a result of the outcome.
Example: Red spots cause chickenpox.
Red spots are a direct result of chickenpox infection.
Reformulated: Chickenpox causes red spots.
Exposure and outcome are relative.
Example: HIV causes AIDS.
There is nothing wrong with this example. However, it is important to remember that one study’s exposure is another’s outcome.
Example 2: Unsafe sex can cause acquisition of HIV.
Your exposure must be measurable.
Example: Having a bad attitude causes wrinkles.
How do you plan to measure having a bad attitude?
Reformulated: Daily frowning causes wrinkles.
The exposure does not need to be a true etiologic cause — they are often markers for other processes or conditions. For example, suppose that you are studying the association between demographics and outcomes following myocardial infarction. Income may be included as a risk factor in your study, and treated as an exposure in the analysis. The absence of money itself does not lead to poor outcomes, but rather serves as a marker for other factors that may be more difficult to measure such as access to health care.