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It may be possible or necessary to access IPD for one or more trials via a data‐sharing repository or platform. For example, there is a reasonable likelihood that trial investigators conducting trials of newer therapies, or that have published results more recently, might have to comply with a funder‐mandated data‐sharing policy requiring that IPD are uploaded to a repository. As described in detail in Section 3.2.2, repositories may not include all trials of relevance to a particular meta‐analysis, or the trials may be distributed across multiple platforms. Therefore, accessing trial IPD in this way can present challenges for the central collection, management, checking and analysis of data, and thereby potentially limit the ability to realise all the advantages generally associated with IPD meta‐analysis projects.⁴⁵ The central research team may need to find ways to work around the issues.

The IPD from a repository may not contain all the variables required to conduct the planned analyses, or these may not be defined in a standard way across different repositories, making data harmonisation difficult. In such circumstances, it may be possible to get an investigator to upload a more appropriate dataset, if the IPD research team can persuade them of the value of doing so. For example, they might emphasise that this is important not only for the current IPD meta‐analysis, but also for subsequent projects that may make use of their trial IPD, and therefore may ultimately prevent data providers receiving repeated requests for further data. However, this would take additional time and resource on the part of both the data provider and the central IPD meta‐analysis research team.

In order to preserve participant confidentiality, often the IPD contained in a repository are subject to a greater degree of de‐identification (Section 4.4.1) than might be required or expected of IPD obtained directly from investigators. This can limit the ability to thoroughly check data (Section 4.5) and assess risk of bias (Section 4.6), and moreover, the opportunity to query any anomalies is lost. Once again, it may be possible to query aspects of the trial, or its associated IPD, through additional contact with trial investigators (including the trial statistician), or to request that they run some validity or risk of bias checks on behalf of the research team. It should be noted though, that for some platforms, the associated data‐sharing agreements require that all queries are mediated through them.

Provided appropriate data are available, are more or less in the same format across trials, and can be downloaded from the relevant repository to use locally, there should not be any restrictions to the analyses of IPD. However, if access to IPD (and therefore data management and analyses) are confined to within a platform, it will only be possible to download results (e.g. regression model parameter estimates and confidence intervals), and this will restrict the meta‐analysis to a two‐stage approach (Part 2).