The efficacy of early intervention programs for children with organic impairment: A quantitative review

The results of studies examining the effectiveness of early intervention for infants and children with organic impairment and developmental delay were reviewed using recently developed quantitative methods that treat the literature review process as a unique type of scientific inquiry. Thirty-eight studies meeting certain predetermined criteria were included in the review. The 38 studies contained a total of 118 statistical hypothesis tests that evaluated the effectiveness of early intervention. An analysis of these tests based on the calculation of effect sizes revealed that subjects receiving early intervention performed better on a wide range of dependent measures than subjects not receiving intervention. The outcomes were found to be related to several design and study characteristics. Larger effect sizes were associated with preexperimental designs, and also with studies in which the internal validity was rated as poor. Several other design variables such as how subjects were assigned to conditions and how the dependent measure was recorded were related to study outcome as measured by effect size. The conclusion was made that an accurate interpretation of the early intervention research literature cannot be made without consideration of specific design variables and study characteristics.