IPD meta-analysis is a specific type of systematic review. Rather than extracting summary (aggregate) data from study publications or from investigators, the original research data are sought directly from the researchers responsible for each study. These data can then be re-analysed centrally and combined, if appropriate, in meta-analyses.
Most IPD meta-analyses are are typically non-Cochrane reviews, carried out and published by collaborative groups. These groups comprise the project team managing the IPD meta-analysis, members of the advisory group (if there is one) and the researchers who contribute their study data for re-analysis.
IPD meta-analyses usually require dedicated staff, with a variety of skills, and would be difficult to do in ‘free time’. They also usually take longer and cost more than a conventional systematic review of published or other summary data.
However, IPD meta-analyses can improve the quality of data and the type of analyses that can be done and produce more reliable results (Stewart and Tierney 2002). For this reason they are considered to be a ‘gold standard’ of systematic review. In fact, IPD meta-analyses have produced definitive answers to clinical questions, which might not have been obtained from summary data.
If you are thinking about doing a Cochrane Review, based on IPD, you may find our resources pages useful along with Chapter 26 of the Cochrane Handbook for Systematic Reviews of Interventions, which was co-written by the convenors, and provides practical guidance for undertaking meta-analyses of individual participant data.