Glossary of main types of bias

Reporting Bias arises when the dissemination of research findings is influenced by the nature and direction of results.

Time Lag Bias is a sub-type of reporting bias. It covers the rapid or delayed publication of research findings, depending on the nature and direction of the results.

Attrition Bias occurs when there are systematic differences between the participants who remain in the study and the participants who withdraw or who are excluded from a study.

Database Bias occurs when the results of indexed studies are systematically different from those of non-indexed studies.

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.

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

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.

Publication Bias occurs when the publication or non-publication of a study depends on the nature and direction of its results

Location Bias occurs when the nature and direction of results are associated with publication in journals with different ease of access of levels or indexing in standard databases.