Typically, the primary outcome in a clinical trial is a measure that is easy
to assess in all patients. In general, "objective" or "hard" outcome
measures that are well-defined and can be observed directly are preferred.
Examples include death (or survival), disease recurrence, change in blood
pressure, etc. More "subjective" outcome measures, such as pain reduction,
quality of life, psychological status, and the like may be equally important
but are more subject to bias when the patient or the investigator knows (or
thinks he knows) which treatment the patient received. The primary outcome
should be one that is clinically relevant; for example, although change in
lipid profile may be an objective, easily measured outcome in a trial for a
lipid lowering agent, it may be of little relevance to patients or clinicians
if there is no concomitant change in risk of heart disease or death.
The outcome must be measurable according to the same criteria, regardless of
the treatment assigned or received. This means that all patients should be
followed in the same way, with the same tests, and at the same intervals
Explicit criteria for determining outcome status must be specified in advance.
It is not enough to design a study to see whether a new treatment, for
example, "reduces blood pressure." It is necessary to specify the criteria
for considering blood pressure to have decreased, i.e., the degree of change
(percent change or absolute change) from baseline level that will be
considered a success.