Glossary Glossary on meta-analysis

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. 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 nonadverse eVect on fetus), sponsorship. 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 eVects of healthcare interventions. These reviews are edited in the Cochrane Database of Systematic Reviews, available on CD-ROM. (More details on it can be found on the internet address www. cochrane.org). Combining: see pooling. Cumulative meta-analysis: A type of metaanalysis in which studies are sequentially pooled by adding each time one new study according to an ordered variable. For instance, if the ordered variable is the year of publication, studies will be ordered by it; then, a pooling analysis will be done every time a new article appears. It shows the evolution of the pooled estimate according to the ordered variable. Other common variables used in cumulative meta-analysis are the study quality, the risk of the outcome in the control group, the size of the diVerence between the groups, and other covariates (for example, mean time to treatment). DerSimonian-Laird’s method: it was first described by Cochran in 1954. It was the first random eVects model and applied to an additive model, based on the risk diVerence and weighted by the inverse of its variance. EVect size: a standardised scale free estimate of the relation between an exposure and an outcome. In a general sense, this term is applied to any measurement of the diVerence in the outcome between the study groups (so relative risk, odds ratio, and risk diVerence can be defined as “eVect sizes”). When the eVect size is applied to measurements of continuous variables (such as mean), its most common estimator is the standardised mean diVerence, which is calculated as the diVerence of means divided by the variability of the measures (the standard deviation). This is mainly useful when there is no common measure to all the studies. Egger’s method: a procedure to detect publication bias. It consists in a simple linear regression of the eVect size in a study divided by its standard error on the inverse of standard error and testing whether the intercept is statistically significant (at p < 0.1). File drawer problem: term coined by Rosenthal to mean the number of statistically non-significant studies (p > 0.05) that remain unpublished. Fixed eVects model: any statistical model assuming homogeneity of eVects across the studies being combined—that is, the true eVect size has a common true value for all studies. In the summary estimate the variance of each study i is taken into account only. Funnel plot: a graphical method to display possible publication bias. It shows the relation between the eVect size of study i and the size of the same study, which can be measured in different ways (standard error of the eVect size, its inverse, sample size, or the number of eVects observed in a study). If there is no publication bias, a typical symmetric funnel shape can be observed. Glass, Gene V: The educational psychologist who coined the term “meta-analysis” in 1976. J Epidemiol Community Health 2001;55:534–536 534