Note: Corrections have been added to this version of the article on 27 May 2011 after first publication online on 10 August 2010.
Sample size planning with the cost constraint for testing superiority and equivalence of two independent groups
Version of Record online: 1 JUN 2011
©2011 The British Psychological Society
British Journal of Mathematical and Statistical Psychology
Volume 64, Issue 3, pages 439–461, November 2011
How to Cite
Guo, J.-H., Chen, H. J. and Luh, W.-M. (2011), Sample size planning with the cost constraint for testing superiority and equivalence of two independent groups. British Journal of Mathematical and Statistical Psychology, 64: 439–461. doi: 10.1348/000711010X512408
- Issue online: 4 OCT 2011
- Version of Record online: 1 JUN 2011
- Received 14 October 2009; revised version received 7 May 2010
The allocation of sufficient participants into different experimental groups for various research purposes under given constraints is an important practical problem faced by researchers. We address the problem of sample size determination between two independent groups for unequal and/or unknown variances when both the power and the differential cost are taken into consideration. We apply the well-known Welch approximate test to derive various sample size allocation ratios by minimizing the total cost or, equivalently, maximizing the statistical power. Two types of hypotheses including superiority/non-inferiority and equivalence of two means are each considered in the process of sample size planning. A simulation study is carried out and the proposed method is validated in terms of Type I error rate and statistical power. As a result, the simulation study reveals that the proposed sample size formulas are very satisfactory under various variances and sample size allocation ratios. Finally, a flowchart, tables, and figures of several sample size allocations are presented for practical reference.