Deciding Who Will Be in a Clinical Study
Because you can't examine the entire population of people with the condition you're studying, you must select a representative sample from that population. You do this by explicitly defining the conditions that determine whether or not a subject is suitable to be in the study.
Inclusion criteria are used during the screening process to identify potential subjects and usually involve subject characteristics that define the population you want to draw conclusions about. A reasonable inclusion criterion for a study of a lipid-lowering treatment would be, "Subject must have a documented diagnosis of hyperlipidemia, defined as Total Cholesterol > 200 mg/dL and LDL > 130 mg/dL at screening."
Exclusion criteria are used to identify subjects for whom participation would be unsafe or those whose participation would compromise the scientific integrity of the study (due to preexisting conditions, an inability to understand instructions, and so on). The following usually appears in the list of exclusion criteria: "The subject is, in the judgment of the investigator, unlikely to be able to understand and comply with the treatment regimen prescribed by the protocol."
Withdrawal criteria describe situations that could arise during the study that would prevent the subject's further participation for safety or other reasons (such as an intolerable adverse reaction or a serious noncompliance). A typical withdrawal criterion may be "The subject has missed two consecutive scheduled clinic visits."