To date, there is little empirical evidence available to allow informed decisions about what is a reasonable and efficient approach to revisiting evidence in Cochrane reviews, although some guidelines do exist (Moher 2007, Shojania 2007a, Shojania 2007b). The Cochrane Collaboration policy is that reviews should either be updated within two years or include a commentary to explain why this is not the case. We define the term ‘update’ in Section 3.2.2. The two-year period starts from the date on which the review was assessed as being up to date (see Section 3.3.2).
In addition to the potential availability of new evidence, other developments may result in the need to revise a review. For example, within the clinical field, better tools or markers for characterizing sub-groups may have been developed, new treatment regimens may be available, or new outcome measures (or refined measurement methods of existing outcomes) may be in use. Furthermore, advances in the methods for conducting a Cochrane review may produce the need to revisit a review.
While conducting a review, authors may be able to judge if relevant research is being published frequently, and therefore may be able to predict and suggest the need for more frequent updating of the review. Alternatively, in some topic areas new data emerge slowly or are unlikely to emerge, and a review prepared many years earlier is still current and valuable. In these cases updating a review every two years may be unnecessary and wasteful (Chapman 2002). Review authors are advised to discuss with their Cochrane Review Group (CRG) if it is felt that their review does not need to be updated at least every two years. The reason why the review is not being updated in line with the Collaboration policy should be stated in the ‘Published notes’ section of the review.