Studies that address the review ASP1517 question, typically clinical trials, should be broadly similar on the population, intervention and comparison groups, but frequently report different outcomes from those chosen by the systematic reviewer. Clinical trialists typically measure numerous outcomes, sometimes in the hundreds. It is likely that these outcomes are different from those chosen by the systematic reviewer; overlap of the chosen outcomes can vary from none to complete. In many cases, the primary outcome of interest to the systematic reviewers may not have been an outcome of interest to the clinical trialists, or may not be reported clearly or consistently in the clinical trial reports or associated documents. Systematic reviewers thus face an important decision: should they choose outcomes to be examined based on what they believe to be important outcomes or based on what they know is reported in the relevant clinical trials ? How systematic reviewers choose outcomes and pre-specify them in systematic review protocols is currently unclear. One view is that, unlike clinical trialists, systematic reviewers should not base outcome choice on sample size/power calculations and Type I error rates. Instead, the objective of medical research should be to draw conclusions based on all sources of available evidence. Systematic reviews, which are often used to inform clinical practice guidelines and policy, could and even should include all the outcomes that patients, clinicians, and policy-makers need to know about. Systematic reviews also allow elucidation of existing research gaps in a given field, for example, when outcomes are not examined in trials and should be. In our view, regardless of who chooses the outcomes to be assessed in a systematic review and how those outcomes are chosen, all outcomes need to be specified completely and clearly if they are to be of use to decision-makers. The objective of our study was to assess the completeness of prespecification and comparability of outcomes in all Cochrane reviews addressing four common eye conditions. We have shown that, if outcome pre-specification in systematic review protocols is judged using recommended standards for clinical trials, then it is largely incomplete. Although completeness appears to have improved somewhat over time, on average, only three of five standard elements of an outcome were pre-specified. Due to largely incomplete outcome pre-specification, a conclusive assessment of comparability in outcome elements across the various protocols per condition was not possible. However, we observed variation in specific metrics and methods of aggregation. Another possible explanation for incomplete pre-specification of outcomes is that choice of outcomes could be influenced by the findings of the clinical trials that would be included in the review. We did not assess the outcomes examined at the level of the clinical trials to determine the likelihood that this occurred, but suggest that doing so may contribute to a better understanding of how review outcomes are chosen. Are they chosen because systematic reviewers consider them the most important outcomes to examine, because they are the outcomes that have been examined in clinical trials, or both? If the review outcomes were chosen purely because they were the outcomes that have been reported in clinical trials, this is troubling because of the possibility of “meta-bias”. We know, for example, that outcomes reported in clinical trials could have been selectively reported because of desirable or undesirable findings. By pre-specifying in the protocol the outcomes to be examined in the review, systematic reviewers minimize the potential for bias.