Dichotomization of Continuous Variables: The Implications for Meta-Analysis

In many studies included in meta-analyses, the independent variable measure, the dependent variable measure, or both, have been artificially dichotomized, attenuating the correlation from its true value and resulting in (a) a downward distortion in the mean correlation and (b) an upward distortion in the apparent real variation of correlations across studies. We present (a) exact corrections for this distortion for the case in which only one of the variables has been dichotomized and (b) methods for making approximate corrections when both variables have been artificially dichotomized. These approximate corrections are shown to be quite accurate for most research data. Methods for weighting the resulting corrected correlations in meta-analysis are presented. These corrections make it possible for meta-analysis to yield approximately unbiased estimates of mean population correlations and their standard deviations despite the initial distortion in the correlations from individual studies.