Brief introduction on studies bias of systematic reviews

Literature bias, which can reduce the validity of a systematic review, comes mainly from the fact that all the related studies can not be collected without exception. The following three types of literature bias are most common. (1) Publication bias. The reason is that some studies can not get published, or the publication has been delayed. (2) Literature retrieval bias. This bias is the result that not all published studies are retrieved during the course of systematic review. (3) Literature inclusion bias. While an improper literature selection strategy is adopted, some useful studies are kicked out by error, leading to such bias. A funnel plot (Y-axis is sample size, X-axis effect) is usually useful to assess literature bias in a systematic review.