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).


METHODS
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.


CONCLUSIONS
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.
Walter
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Department of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ontario, Canada. walter@mcmaster.ca
Yao
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D003627:Data Interpretation, Statistical; D019317:Evidence-Based Medicine / Q000706:statistics & numerical data*; D006801:Humans; D015201:Meta-Analysis as Topic; D016011:Normal Distribution; D016032:Randomized Controlled Trials as Topic; D012016:Reference Values; D018401:Sample Size; D016896:Treatment Outcome