"First - there is bias in everything. The methodologies developed and used to conduct systematic reviews work to reduce bias as much as possible but there has never been a promise, nor should someone expect, that bias be eliminated entirely. The goal to reduce bias is why we search more than one database and search thoroughly to locate as much available work that has been done relating to the particular question posed by the review team. With that in mind, an appropriate question and resulting response to the peer reviewer would be how was Cochrane used in the review? It seems likely that unless the review was limited entirely to Cochrane databases and methods, the inclusion of Cochrane databases as one resource and one source of methodology out of a few databases and methodologies, would not spoil the entire review with an unreasonable level of bias."
Types of bias include: