Attrition bias. To determine whether attrition threatens the internal validity of the study
we compared the characteristics of those remaining in the study across different treatment
conditions (Cook &Campbell, 1979). Findings indicated no condition x attrition status
interaction for any of the primary variables (Gates-MacGinitie, TOWRE, AIMSweb), suggesting
that study attriters did not differ significantly at time 1 across conditions.