Reviews have often been exclusively based on studies published in English. For example, among 36 meta-analyses reported in leading English-language general medicine journals from 1991 to 1993, 26 (72%) had restricted their search to studies reported in English (Grégoire 1995). This trend may be changing, with a recent review of 300 systematic reviews finding approximately 16% of reviews limited to trials published in English; systematic reviews published in paper-based journals were more likely than Cochrane reviews to report limiting their search to trials published in English (Moher 2007). In addition, of reviews with a therapeutic focus, Cochrane reviews were more likely than non-Cochrane reviews to report having no language restrictions (62% vs. 26%) (Moher 2007).
Investigators working in a non-English speaking country will publish some of their work in local journals (Dickersin 1994). It is conceivable that authors are more likely to report in an international, English-language journal if results are positive whereas negative findings are published in a local journal. This was demonstrated for the German-language literature (Egger 1997b).
Bias could thus be introduced in reviews exclusively based on English-language reports (Grégoire 1995, Moher 1996). However, the research examining this issue is conflicting. In a study of 50 reviews that employed comprehensive literature searches and included both English and non-English-language trials, Jüni et al reported that non-English trials were more likely to produce significant results at P<0.05, while estimates of intervention effects were, on average, 16% (95% CI 3% to 26%) more beneficial in non-English-language trials than in English-language trials (Jüni 2002). Conversely, Moher and colleagues examined the effect of inclusion or exclusion of English-language trials in two studies of meta-analyses and found, overall, that the exclusion of trials reported in a language other than English did not significantly affect the results of the meta-analyses (Moher 2003). These results were similar when the analysis was limited to meta-analyses of trials of conventional medicines. When the analyses were conducted separately for meta-analyses of trials of complementary and alternative medicines, however, the effect size of meta-analyses was significantly decreased by excluding reports in languages other than English (Moher 2003).
The extent and effects of language bias may have diminished recently because of the shift towards publication of studies in English. In 2006, Galandi et al. reported a dramatic decline in the number of randomized trials published in German-language healthcare journals: with fewer than two randomized trials published per journal and year after 1999 (Galandi 2006). While the potential impact of studies published in languages other than English in a meta-analysis may be minimal, it is difficult to predict in which cases this exclusion may bias a systematic review. Review authors may want to search without language restrictions and decisions about including reports from languages other than English may need to be taken on a case-by-case basis.