Location bias in controlled clinical trials of complementary/alternative therapies

To systematically investigate location bias of controlled clinical trials in complementary/alternative medicine (CAM).

Literature searches were performed to identify systematic reviews and meta-analyses, which were used to retrieve controlled clinical trials. Trials were categorised by whether they appeared in CAM-journals or mainstream medical (MM)-journals, and by their direction of outcome, methodological quality, and sample size.

351 trials were analysed. A predominance of positive trials was seen in non-impact factor CAM- and MM-journals, (58) / (78) (74%) and (76) / (102) (75%) respectively, and also in low impact factor CAM- and MM-journals. In high impact factor MM-journals there were equal numbers of positive and negative trials, a distribution significantly (P < 0.05) different from all other journal categories. Quality scores were significantly lower for positive than negative trials in non-impact factor CAM-journals (P < 0.02). A similar trend was seen in low-impact factor CAM journals, but not to a level of significance (P = 0.06). There were no significant differences between quality scores of positive and negative trials published in MM-journals, except for high impact factor journals, in which positive trials had significantly lower scores than negative trials (P = 0.048). There was no difference between positive and negative trials in any category in terms of sample size.

More positive than negative trials of complementary therapies are published, except in high-impact factor MM-journals. In non-impact factor CAM-journals positive studies were of poorer methodological quality than the corresponding negative studies. This was not the case in MM-journals which published on a wider range of therapies, except in those with high impact factors. Thus location of trials in terms of journal type and impact factor should be taken into account when the literature on complementary therapies is being examined.