The National Institutes of
Health has defined a clinical trial as
a research study in which one or more
human participants are prospectively assigned to one or more interventions
(which may include placebo or other control) to evaluate the effects of those interventions
on health-related biomedical or behavioral outcomes. In terms of strength of study design, the randomized clinical trial is considered
the strongest design to evaluate the cause-and-effect relationship of an
intervention.
Clinical trials can be
classified as either therapeutic or preventive.
Therapeutic trials
investigate the effect of a treatment (independent variable) on an outcome
(dependent variable).
Preventive trials assess if a treatment (independent variable) is effective in reducing
the risk of developing a condition or disease (dependent variable).
Various experimental designs
exist and I will be discussing different designs in future posts. As for now, I will be discussing different
types of clinical trial designs.
The “gold standard” of
experimental designs is the randomized clinical trial (RCT). The RCT design is also referred to as a
pretest-posttest control group design. This
design is used to compare two or more groups that are created by random
assignment, where one group receives the experimental treatment and the other
group(s) serve as a control or comparison.
The different groups are sometimes referred to as treatment arms. All groups
undergo pre-testing before receiving treatment and post-testing
afterwards. During pre- and
post-testing, outcomes data are collected from the study participants. If the experimental group experiences a
greater change in outcomes between pre- and post-testing than the control
group, one can infer that the treatment caused an effect.
The information in previous
posts on sampling,
validity
of intervention study design, and threats
to study validity help to explain how the RCT is a strong study design. Below is a flowchart that illustrates a RCT.
http://www.consort-statement.org/consort-statement/flow-diagram
The posttest-only control group design is the same as the pretest-posttest
control group, except pre-testing does not occur in the experimental group nor control/comparison
group. In some situations, pre-testing
is not possible.
Factorial designs are clinical trials that test the effect of more than one treatment, where
potential interactions between the treatments are evaluated.
Randomized block designs are clinical trials where investigators wish to
control for a possible extraneous variable that may influence differences
between groups. In such cases,
homogeneous blocks of study participants.
For example, investigators may think that gender could influence the
differences in effects between groups.
Each gender group could be randomly assigned to either the experimental
group or the control group. If 24
participants are included in the study (12 male and 12 female), the males could
randomly assigned to groups (6 males in each group) and the female could be
randomly assigned to groups (6 females in each group). Gender can now be considered an independent
variable and differences between males and females can be evaluated.
For more information about clinical
trial designs, click on the following link.
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