As part of a systematic review a risk of bias analysis should be carried out for all included studies.
There are a number of different types of bias including the following which can affect the quality of a study. The following are the main types of bias in relation to randomised controlled trials:
For an explanation of these and other different kinds of bias and the consequences of such bias please refer to the resources below:
Australian Government National Health and Medical Research Council Assessing the risk of bias: Draft module - This contains a useful list of different tools to assess the risk of bias for different types of studies