Definition of terms for clinical trials
and scientific experiments
- Sample size:
- The number of patients or experimental units required for the trial.
- The probability that a clinical trial will have a significant(positive) result, that is have a p-value of less than the specified significance level(usually 5%). This probability is computed under the assumption that the treatment difference or
strength of association equals the minimal detectable difference.
- Minimal detectable difference:
- The smallest difference between the treatments or strength of association that you wish to be able to detect. In clinical trials this is the smallest difference
that you believe would be clinically important and biologically plausible. In a study of association it is the smallest change in the dependent(outcome variable, response), per unit change in the independent(input variable, covariate) that is plausible.
- Parallel design:
A parallel designed clinical trial compares the results of a treatment on
two separate groups of patients. The sample size calculated for a parallel
design can be used for any study where two groups are being compared.
- Crossover study:
A crossover study compares the results of a two treatment on
the same group of patients. The sample size calculated for a crossover study
can also be used for a study that compares the value of
a variable after treatment with it's value before treatment. The standard
deviation of the outcome variable is expressed as either
the within patient standard deviation or the standard deviation
of the difference. The former is the standard deviation of repeated
observations in the same individual and the latter is the standard deviation
of the difference between two measurements in the same individual.
- Study to find an association:
A study to find an association determines if a variable, the dependent variable, is affected by another, the independent variable. For instance, a study to
determine whether blood pressure is affected by salt intake.
The outcome of the study is a variable with two values, usually treatment success or treatment failure.
- The outcome of the study is a continuous measurement.
- Time to an Event:
- The outcome of the study is a time, such as the time to death, or relapse. Some patients will not have been observed to relapse. These observations are said to be censored .
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Last Updated Wed. April 8 1998
David A. Schoenfeld, Ph.D.,
With support from the
Massachusetts General Hospital Mallinckrodt
General Clinical Research Center,
Research Resources Division, National Institutes of Health,
General Clinical Research Center Program.