Exploring changes over time and characteristics associated with data retrieval across individual participant data meta-analyses: systematic review

OBJECTIVE To investigate whether the success rate of retrieving individual participant data (IPD) for use in IPD meta-analyses has increased over time, and to explore the characteristics associated with IPD retrieval. DESIGN Systematic review of published IPD meta-analyses, supplemented by a reflection of the Cochrane Epilepsy Group's 20 years' experience of requesting IPD. DATA SOURCES Medline, CENTRAL, Scopus, Web of Science, CINAHL Plus, and PsycINFO. ELIGIBILITY CRITERIA FOR STUDY SELECTION IPD meta-analyses of studies of all designs and all clinical areas published in English. RESULTS 760 IPD meta-analyses which identified studies by systematic methods that had been published between 1987 and 2015 were included. Only 188 (25%) of these IPD meta-analyses retrieved 100% of the eligible IPD for analysis, with 324 (43%) of these IPD meta-analyses retrieving 80% or more of relevant IPD. There is insufficient evidence to suggest that IPD retrieval rates have improved over time. IPD meta-analyses that included only randomised trials, had an authorship policy, included fewer eligible participants, and were conducted outside of the Cochrane Database of Systematic Reviews were associated with a high or complete IPD retrieval rate. There was no association between the source of funding of the IPD meta-analyses and IPD retrieval rate. The IPD retrieval rate of the Cochrane Epilepsy Group has declined from 83% (up to 2005) to 65% (between 2012 and 2015) and the reported reasons for lack of data availability have changed in recent years. CONCLUSIONS IPD meta-analyses are considered to be the gold standard for the synthesis of data from clinical research studies; however, only 25% of published IPD meta-analyses have had access to all IPD.