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Glossary on meta-analysis
  1. M Delgado-Rodríguez
  1. Division of of Preventive Medicine and Public Health, Building B-3, University of Jaen, 23071-Jaén, Spain
  1. Professor Delgado-Rodríguez (mdelgado{at}

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Bias, language: the systematic error introduced when the search of potential studies to be included in a meta-analysis is focused, in general, in one language (mainly English). Given that studies published in English are more widely read, it is possible that significant results of studies carried out in non-English countries are more published in English journals than in journals written in other languages.1

Bias, search: it is related to the language bias. It is the systematic error introduced when the search of studies is centred in just one database (for example, Medline). Journals written in English are overrepresented in Medline; furthermore, the journals from the country (and from neighbouring countries or either countries of similar language or culture) where the database is done are also represented in excess. It is recommended to consult more than one database (for example, Medline and Embase), supplemented by a hand search of the references of each publication collected.

Bias, publication: bias produced when the published studies do not represent adequately all the studies carried out on a specific topic. Many facts can origin this bias, although the best known is the trend to publish statistically significant (p < 0.05) or clinically relevant (high magnitude albeit non-significant) results. Other variables influencing publication bias are sample size (more in small studies), type of design (less in well conducted randomised controlled trials), funding, conflict of interest, prejudice against an observed association (for example, cocaine consumption and non-adverse effect on fetus), sponsorship.2 3

Cochrane Collaboration: an international organisation, named after Archibald Cochrane, which aims to help people make well informed decisions about healthcare by preparing, disseminating, and continuously updating systematic reviews of controlled clinical trials on the effects of healthcare interventions. These reviews are edited in the Cochrane Database of Systematic Reviews, available on CD-ROM.4 …

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