Effect sizes can be calculated for studies reporting ranges for outcome variables in systematic reviews

OBJECTIVE: To develop a method by which studies reporting the ranges (or maxima and minima) of observed outcomes can be included in systematic reviews, along with other studies reporting in the more usual way by using standard deviations (SDs). STUDY DESIGN AND SETTING: An approach is proposed to allow a numerical conversion of a reported range from a continuous outcome into an equivalent SD. RESULTS: The SD is estimated from the observed range times an appropriate conversion factor. Two examples (the first concerning a patient education program on adherence to drug treatment for rheumatoid arthritis, and the second investigating if cognitive behavior therapy could improve adherence to antiretroviral therapy and then lead to suppression of human immunodeficiency virus replication) demonstrate the calculations. CONCLUSION: This note provides a simple method to allow studies that report outcome variability in terms of ranges to be included in systematic reviews by conversion to equivalent SDs. The method is entirely valid if the outcome variable is normally distributed, but for nonnormal data, some caution may be needed.