Using an intraclass correlation to measure consistency across subjects

Fit a multilevel model

MIXED Y 
  /FIXED=  | SSTYPE(3)
  /METHOD=REML
  /PRINT=SOLUTION
  /RANDOM=intercept | SUBJECT(participant) COVTYPE(VC).

The intraclass correlation which measures the consistency of performance across version of the participants equals

Variance Component for Participant / 
 [(Variance Component for Participant) + (Variance Component of Residual)]

This can also be computed from a one way repeated ANOVA if there are the same number of repetitions (e.g. time) per participant.