Data collection in a clinical trial proceeds according to criteria that are
explicit and specified in advance. It is essential that the same measurement
methods, schedule, and criteria apply to all patients in all treatment groups.
Baseline characteristics should be measured before patients are randomized so
that knowledge of the treatment assignment cannot influence reporting of
baseline characteristics. It is important to monitor all patients with
respect to the treatment, both the treatment that was assigned as well as
the treatment eventually received (as all patients may not be treated
according to the random assignment). Some patients may refuse the assigned
treatment; in a pharmaceutical trial, some patients may not comply fully,
taking only a portion of the assigned medication. Methods such as pill
counts (or more high-tech approaches) are useful in assessing the level of
compliance with the assigned treatment.
The "outcome" is the factor on which the assessment of treatment efficacy is
based. In order to determine whether a new treatment is "better", the
criterion on which it will be judged, the primary outcome, must be specified
in advance. Similarly, other effects, both desirable and undesirable, are
generally measured in order to gain a more complete picture of the benefits
and risks of the new treatment.