10.2.2.2  Location bias

Research suggests that various factors related to the accessibility of study results are associated with effect sizes in trials.  For example, in a series of trials in the field of complementary and alternative medicine, Pittler and colleagues examined the relationship between trial outcome, methodological quality and sample size with characteristics of the journals of publication of these trials (Pittler 2000). They found that trials published in low or non-impact factor journals were more likely to report significant results than those published in high-impact mainstream medical journals and that the quality of the trials was also associated with the journal of publication.  Similarly, some studies suggest that trials published in English language journals are more likely to show strong significant effects than those published in non-English language journals (Egger 1997b), however this has not been shown consistently (Moher 2000, Jüni 2002, Pham 2005); see Section 10.2.2.4.

 

The term ‘location bias’ is also used to refer to the accessibility of studies based on variable indexing in electronic databases.  Depending on the clinical question, choices regarding which databases to search may bias the effect estimate in a meta-analysis.  For example, one study found that trials published in journals that were not indexed in MEDLINE might show a more beneficial effect than trials published in MEDLINE-indexed journals (Egger 2003).  Another study of 61 meta-analyses found that, in general, trials published in journals indexed in EMBASE but not in MEDLINE reported smaller estimates of effect than those indexed in MEDLINE, but that the risk of bias may be minor, given the lower prevalence of the EMBASE unique trials (Sampson 2003).  As above, these findings may vary substantially with the clinical topic being examined.

 

A final form of location bias is regional or developed country bias.  Research supporting the evidence of this bias suggests that studies published in certain countries may be more likely than others to produce research showing significant effects of interventions. Vickers and colleagues demonstrated the potential existence of this bias (Vickers 1998).