For some of the largest ongoing critical care trials, many clinicians regard prior probabilities as low and consider that plausible effects on absolute mortality are less than 5%.

Further work is needed to determine whether pooled estimates obtained by surveying clinicians are replicable and accurate or whether other methods of estimating prior probability are preferred.

Estimates provided by individual clinicians varied from 0% to 100% for most trials, with a median estimate of 15% (IQR 10-20%).

The median largest absolute mortality reduction considered plausible was 4.5% (IQR 3.5-5%), compared with a median absolute mortality reduction used in sample size calculations of 5% (IQR 3.6-10%) (P = 0.27).

We conducted a systematic review of clinical trial registries in September 2015 to identify ongoing critical care medicine trials where landmark mortality was the primary outcome, followed by a clinician survey to obtain opinions about ten large trials.

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