The Effectiveness of a Parents as Teachers Home Visitation Program on School Readiness: An Application of Complier Average Causal Effect Analysis.

Purpose: The purpose of this study is to evaluate an encouragement trial of a Parents as Teachers (PAT) home visitation intervention on the school readiness of preschool children using an innovative analysis to address issues of selective enrollment.

Method: Families were given the opportunity to enroll in a PAT program through a randomized lottery. The PAT program is assessed using standardized measures of school readiness before and after the two-year program. A comparison of three different analyses is used to evaluate the program - Average Treatment Effect (ATE) analysis, Intent-to-Treat (ITT) analysis, and Complier Average Causal Effect (CACE) analysis. CACE is an innovative analysis developed specifically to diagnose bias arising from selective enrollment in the context of an encouragement trial.

Result: All three analyses (ATE, ITT, and CACE) provide statistically significant evidence of an effective PAT program. However, the effect sizes for the CACE analysis are over twice as large as the other two analyses. The Cohen's D for CACE is .934 compared to .424 for ATE and .381 for ITT.

Conclusion: This study provides evidence of an effective PAT program. The comparison of ATE, ITT, and CACE analyses reveals the potential for meaningful under-reporting of the program's impact if selective enrollment is ignored. CACE analysis demonstrates how selective enrollment can bias evaluations of home visitation interventions in general.