TABLE 1.

Major differences between fixed versus random effects approaches to analyzing therapist effects

Dimension Compared	Approach to Analyzing Therapist Effects
	Fixed Therapist Effects	Random Therapist Effects
Scope of inference (i.e., the results may be generalized to…)	The specific therapists in the trial	The population of therapists
Assumptions about the sample of therapists in the trial	None	The therapists are a representative sample from a specified population
Intratherapist correlation (i.e., correlation of patient outcomes within therapists)	Ignored (assumed to be zero)	Estimated in the analysis (may be different from zero)
Residual variance	Equal across patients (homoscedasticity)	May be unequal across patients (heteroscedasticity)
Recommended minimum sample size of therapists	At least two	At least five