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New guidance published yesterday by a team led by NDORMS Associate Professor Jonathan Cook will help researchers recruit the right number of people for clinical trials and answer key research questions.

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How do researchers decide how many people they need to recruit into their clinical trials? Until recently, there was surprisingly little practical guidance available to help them. But a team led by NDORMS Associate Professor Jonathan Cook yesterday published new guidance to help researchers decide the sample size needed for their trials and explain their decisions in later academic papers.

The number of people needed in a trial is based on how big a difference the treatment being investigated is expected to make or the size of difference that patients and clinicians would consider important. This is otherwise known as the target difference.

Although there are various methods in use to set this target difference, it turns out that not all of them are a good idea. The DELTA2 guidance stresses that the target difference should be both realistic – something that a trial can plausibly see – and important – a difference that patients and clinicians alike consider enough to change clinical practice.

"Getting the target difference and sample size is a critical part of designing a clinical trial," says Associate Prof Cook.

"Recruit too few people, and the study may not be able to draw firm conclusions about whether the new treatment actually works. Recruit more people than are actually needed, and those extra participants are needlessly involved in your trial, wasting their time, preventing them from enrolling in another study, and possibly spending valuable resource for no added information."

The guidance also helps researchers to clearly explain how their sample size was decided on. Funders reviewing grant applications and other researchers reading a study's protocol or results paper will then have enough information to understand and replicate the decisions, which is crucial for judging the trial's results themselves.

In a commentary piece published by Trials, Professor Melanie Bell of the University of Arizona highlights the potential benefit of this work: "The DELTA2 guidance will help to reduce the guesswork, and this should translate into better health research."

To reach as many researchers as possible, the DELTA2 guidance has been published simultaneously in the BMJ and BMC's Trials, with an accompanying blog post by the authors on the BioMed Central Blog Network. Trials has also created an article series on this key topic.

"My hope is that the DELTA2 guidance will provide the practical help for researchers that I wish I had had when I started out as a statistician working on clinical trials carrying out sample size calculations," says Associate Prof Cook. "Ideally funders and journals will recommend the guidance to the researchers they work with."

The DELTA2 project was funded by the Medical Research Council (MRC)/National Institute for Health Research (NIHR) UK Methodology Research Panel in response to an open commissioned call for an Effect Size Methodology State-of-the-art Workshop. The guidance was produced by a team of researchers from the UK, USA, and Canada.