Attrition Bias occurs when a study is performed over a period of time in which participants can drop out or withdraw themselves from the study for any number of reasons, if those who drop out are systematically different from those who remain in the study bias will be present.

Database Bias occurs when there is biased indexing of published studies in literature databases (Felson 1992). The literature search will be biased when it is based on a database in which the results of indexed studies are systematically different from those of non-indexed studies (Song 2000).

Funding Biases are the biases in the design, outcome, and reporting of industry sponsored research in order to show that a drug shows a favourable outcome (Lexchin 2003).

Grey literature Bias occurs when the results reported in journal articles are systematically different from those presented in reports, working papers, dissertations, or conference abstracts (Song 2000).

Language Bias occurs when languages of publication depend on the direction and strength of the study results (Gregoire 1995).

Outcome Reporting Bias occurs when a study in which multiple outcomes were measured reports only those that are significant, ignoring those that were insignificant or unfavourable (Song 2000).

Publication Bias or positive results bias, basic publication bias is the tendency on the part of the investigators, reviewers, and editors to submit or accept manuscripts for publication based on the direction or strength of the study findings (Dickersin 1990).

Location Bias occurs when the country of publication is associated with the strength or direction of research findings (Song 2000).

Study Quality Bias occurs when studies of lower or higher quality are associated with positive or favorable results.