Term Definition
Common interventionist or facilitator

the physical or virtual interventionist that delivers at least a part of the intervention to more than one member or participant.

Cohort design

the same members or participants are observed at each measurement occasion.


the study conditions reflect the levels of the intervention, usually intervention vs. control in a study with two study conditions. Study condition is equivalent to study arm.

Cross-sectional design

different members or participants are observed at each measurement occasion.

the hypothesized intervention effect, in units corresponding to the scale of the variance.


degrees of freedom.


the units of randomization, often called groups or clusters, nested within the study conditions or arms.


group-randomized trial, also called a cluster-randomized trial, where the units of assignment are groups or clusters and the units of observation are members of those groups or clusters.


intraclass correlation coefficient or intracluster correlation coefficient.

Intraclass correlation

the expected average correlation for the outcome among members of the same group or cluster at the time of the analysis, usually at the posttest. It is also defined as the proportion of variance in the outcome attributable to group membership at the time of the analysis, usually at the posttest.


individually-randomized group treatment trial, also called a partially clustered design, where individuals are randomized to study conditions but receive at least some of their intervention in groups or from a common interventionist or facilitator.


the units of observation, also called participants, nested within groups.

Nested factor

a factor is nested if it appears within only one level of a factor higher in the hierarchy. Subjects are nested within groups or clusters if they appear in only one group or cluster. Groups are nested within study conditions if they appear in only one study condition.

Net difference

the difference between two study conditions over two points in time.


the probability of correctly rejecting a false null hypothesis.


randomized controlled trial, or randomized clinical trial, where the units of assignment and observation are the study participants, and where participants have no interaction with each other or with a common interventionist or facilitator post-randomization.

Simple difference

the difference between the two study conditions at one point in time.

Stepped wedge design

the stepped wedge design is a crossover GRT design, in which each group is observed initially under the control condition and eventually under the intervention condition.


the stratification levels, e.g., high vs. low.


the measurement occasions or periods, usually pretest and posttest.

Type I error rate

the probability of making a type I error, which is to incorrectly reject the null hypothesis of no intervention effect.

Type II error rate

the probability of failing to reject a false null hypothesis.


the expected variance of the outcome at the time of the analysis, in either standardized units (=1) or in the natural units of the outcome (e.g., lbs. or mm Hg).