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The use of one- versus two-tailed tests to evaluate prevention programs
Chris Ringwalt1, M J Paschall, Dennis Gorman
1Pacific Institute for Research and Evaluation, Chapel Hill, NC, USA. ringwalt@pire.org
Evaluation & the Health Professions
|December 9, 2010
Summary
Statistical significance testing in program evaluations often incorrectly uses one-tailed tests. Researchers should avoid these tests due to increased Type I error risk and report exact p-values instead.
Area of Science:
- Program evaluation methodology
- Statistical inference in social sciences
Background:
- Significance testing is crucial for evaluating program effectiveness.
- The use of one-tailed versus two-tailed tests in research has been debated.
- Current consensus suggests one-tailed tests are rarely appropriate in program evaluation.
Purpose of the Study:
- To assess the prevalence of one-tailed significance tests in published school-based drug prevention program evaluations.
- To examine the justifications provided for using one-tailed tests.
- To advocate for standardized and rigorous statistical practices in program evaluation.
Main Methods:
- Systematic review of 85 published evaluations of school-based drug prevention curricula.
- Analysis of statistical methods employed, specifically the use of one-tailed and two-tailed significance tests.
- Examination of reported criteria for determining statistical significance.
Main Results:
- 20% of reviewed evaluations utilized one-tailed significance tests.
- An additional 4% within the one-tailed subgroup also employed two-tailed tests.
- Many publications omitted details on statistical tests or relied on effect sizes/confidence intervals.
Conclusions:
- One-tailed tests should be entirely avoided in program evaluation due to heightened risk of Type I errors and comparability issues.
- The conventional alpha level of .05 is arbitrary; exact p-values should be reported for transparency and consistency.
- Standardizing statistical reporting enhances the reliability and interpretability of program evaluation findings.
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