The four basic types of evaluation: clinical reviews, clinical trials, program reviews, and program trials.

Four basic types of evaluation, each appropriate in a distinctive situation, are the clinical review and the clinical trial, which are concerned with the care of the individual patient, and the program review and program trial, which deal with programs or services directed at groups or populations. Evaluative reviews are primarily motivated by concern with the welfare of the specific population served, and they appraise specific activities in specific settings as a basis for decisions concerning these activities. Clinical and program trials aim to generate knowledge of more general applicability, especially concerning causal relationships between care and outcomes. The types of evaluation differ in the questions they pose and in the methods used to answer them. Failure to draw a distinction between program reviews and program trials is a frequent cause of wasteful or unhelpful evaluative studies.