In planning the search process it is necessary to take into account what other searching has already been undertaken to avoid unnecessary duplication of effort. For example, considerable efforts over the years have gone into searching MEDLINE and EMBASE and incorporating reports of trials from these two major international databases into the Cochrane Central Register of Controlled Trials (CENTRAL). It is necessary, therefore, that any additional searching for a specific review should take into account what has gone before.  Figure 6.3.a illustrates the contents of CENTRAL.