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Estimating the Standardized Mean Difference in Intervention Studies

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  • Robert L. Fowler

Abstract

Methods for approximating confidence intervals for the population standardized mean difference, δ , were evaluated analytically in small samples. A procedure based on the large sample approximation of the distribution of the sample standardized mean difference d was quite accurate over most of the range .25 ≤ δ ≤1.5 and 6 ≤ N ≤ 40 examined. Use of Hedges’s adjustment to d yielded confidence intervals that were slightly conservative, whereas the unadjusted d produced intervals that were somewhat liberal. An empirically determined simple linear adjustment to d demonstrated the most consistent precision when used to construct confidence intervals for δ , having a maximum error of less than 2% of the nominal level of confidence over the effect size range of most interest to behavioral scientists.

Suggested Citation

  • Robert L. Fowler, 1988. "Estimating the Standardized Mean Difference in Intervention Studies," Journal of Educational and Behavioral Statistics, , vol. 13(4), pages 337-350, December.
    • Handle: RePEc:sae:jedbes:v:13:y:1988:i:4:p:337-350
    DOI: 10.3102/10769986013004337
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