Statistical Conclusion Validity of Early Intervention Research with Handicapped Children

The statistical conclusion validity of early intervention research studies was examined by conducting a post hoc power analysis of 484 statistical tests from 49 early intervention articles. Statistical power determinations were made based on Cohen's (1977) criteria for small, medium, and large effect sizes. The analysis revealed that the median power to detect small, medium, and large effect sizes ranged from 0.08 to 0.46. Four percent of early intervention studies had adequate power (.80 or greater) to detect medium intervention effects and 18% to detect large intervention effects. The power values suggest poor statistical conclusion validity in the analyzed research and should alert investigators to the possibility of Type II experimental errors in the early intervention research literature. The argument is made that low statistical conclusion validity has practical consequences in relation to program evaluation and cost-effectiveness determinations.