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Thus, a carefully considered target difference is key. The sample size required is highly sensitive to the chosen target difference: a halving of the difference in means results in quadruple the required sample size for a balanced two-arm randomized controlled trial (RCT), for example. The target difference is a key quantity in sample size calculation, and is the most difficult to determine, as most other quantities are fixed (e.g., type I error rate = 0.05, power = 80 or 90%) or are parameters that can be estimated (standard deviation, control group event proportion). It can be expressed as the difference in means or proportions, odds ratio, relative risk or hazard ratio between arms. What follows is likely to be a muddled conversation about important differences (what is important?), plausibility (how do we determine that?) and budget (we cannot afford any more than n = x).ĭespite the paramount importance of an a-priori sample size calculation, until now there has not been comprehensive guidance in specifying the target effect size, or difference. The DELTA researchers have developed a set of comprehensive guidance documents that are welcome and will almost certainly improve the way that trials are designed and reported.īiostatistician: What is the difference that we should base our sample size calculations on?īiostatistician: Uh…you are supposed to tell me. It is hoped that this will contribute not only to better powered studies, but better reporting and reproducibility and thinking about what a trial should be designed to achieve.
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It gives recommendations on how to find appropriate differences, conduct the sample size calculation(s) and how to report these in grant applications, protocols and manuscripts. The DELTA 2 guidance stresses specifying important and realistic differences, and undertaking sensitivity analyses in calculating sample sizes. In addition to surveying methods that researchers are using in practice, the research team met with various experts (statisticians, methodologists, clinicians and funders) reviewed guidelines from funding agencies and reviewed recent methodological literature. This is a commentary on a collection of papers from two important projects, DELTA (Difference ELicitation in TriAls) and DELTA 2 that aim to provide evidence-based guidance on systematically determining the target effect size, or difference and the resultant sample sizes for trials. However, until now, there has not been comprehensive guidance on how to specify this effect. To ensure that the trial provides good evidence to answer the trial’s research question, the target effect size (difference in means or proportions, odds ratio, relative risk or hazard ratio between trial arms) must be specified under the conventional approach to determining the sample size.
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Sample size calculations are central to the design of health research trials.