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Having concluded that an IPD meta‐analysis is warranted (Section 2.6) and used the development of a project scope to work through design and organisational issues, it is important to assess whether the project is likely to be feasible – in particular, whether sufficient IPD will be available to support credible analysis.

It is worth bearing in mind that it is not always possible or necessary to obtain IPD from every eligible trial. IPD from some older trials may have been lost or destroyed, important participant‐level outcomes or covariates may not have been collected in some trials, and not all investigators may wish to collaborate. Deciding how many trials are sufficient for the IPD meta‐analysis project to proceed is not straightforward, and depends not only on the number of trials likely to be available, but on their size and which outcome and covariate data are available, as these influence the statistical power that the IPD meta‐analysis will have (Chapter 12). For example, when examining participant‐level covariates, the within‐trial variability of covariate values is also influential toward the power of an IPD meta‐analysis, in additional to the total number of participants and events.³³