To guard against bias, the studies were analyzed independently. One investigator selected the articles for inclusion according to pre-established criteria. Another two investigators, who had no knowledge of the financial sponsors, and who were not told the article's author, title or journal of publication, classified the articles' conclusions as "favorable," "neutral" or "unfavorable." A fourth investigator, who had no knowledge of the conclusions, determined the funding source (22 percent were funded entirely by industry, 47 percent had no industry funding, and 32 percent had mixed funding) and classified articles as to whether a favorable finding would be beneficial, negative or neutral to its funder's financial interests.
When analyzed statistically, article conclusions were significantly related to funding source. Interventional studies with all-industry funding were much less likely to have unfavorable conclusions than those with no industry funding (0 vs. 37 percent). Among all types of studies, comparing all-industry versus no-industry funding, the odds ratio for having a favorable versus unfavorable conclusion was 4.37, increasing to 7.61 when beverage type, publication year and examination of authors' personal conflicts of interest were taken into account.
The researchers note that during their five-year study, the overall proportion of papers declaring their funding sources increased, as journals tightened their disclosure requirements. However, taking publication year into consideration didn't weaken the study's findings. "This isn't an effect that's disappearing," Ludwig says.