Читать книгу Evidence in Medicine - Iain K. Crombie - Страница 24

Before recruiting patients, many trials report their detailed methods in an international trial register. Several international registers have been established (e.g. ClinicalTrials.gov and the ISRCTN registry) [19, 20]. Many researchers also publish their study protocols in a medical journal. These sources allow other researchers to compare the outcome measures that were initially specified with those that are presented in the publication of the trial results. A review of outcome reporting in high quality neurology journals found that in 180 trials, 21% of the specified primary outcomes had been omitted, 6% of primary outcomes were demoted to secondary outcomes and 34% of trials added previously unmentioned primary outcomes [21]. A similar pattern was seen in trials published in haematology journals where 40% of primary outcomes had been omitted, 25% of primary outcomes were demoted and many new outcomes were added [22]. The evidence is clear that in trials across the medical specialties, primary outcomes are frequently changed [23–26].

Outcomes may be changed for good reasons, such as replacing a difficult to measure outcome with a more amenable one. But there may be other motives. Several studies have shown that the effect of substituting outcomes favours the publication of positive findings [21, 26]; that is when a non‐significant primary outcome is demoted and a significant secondary one is promoted to primary outcome. Compared to trials with unchanged outcomes, those with substituted outcomes report an increased effect size [27].

One study explored why authors had omitted or changed outcomes [28]; often this was because the researchers thought that a non‐significant result was not interesting. A review of such studies found that a preference for positive finding, and a poor or flexible research design, were the reasons most commonly mentioned for switching outcomes [29]. It seems likely that outcomes are sometimes changed based on the findings from an initial analysis.