Recognizing vested interests and spin in systematic reviews
What if the reviewers have other interests that might affect the conduct or interpretation of their review? Perhaps the reviewers have received money from the company that made the new treatment being tested. When assessing the evidence for an effect of evening primrose oil on eczema, reviewers who were associated with the manufacturer reached far more enthusiastic conclusions about the treatment than those with no such commercial interest.
However, commercial interests are not alone in leading to biased reviews. We all have prejudices that can do this – researchers, health professionals, and patients alike.
Disappointingly, people with vested interests sometimes exploit biases to make treatments look as if they are better than they really are. 
This happens when some researchers – usually but not always for commercial reasons – deliberately ignore existing evidence. They design, analyze, and report research to paint their own results for a particular treatment in a favourable light.
This is what happened in the 1990s when the manufacturer of the anti-depressant drug Seroxat (paroxetine) withheld important evidence suggesting that, in adolescents, the drug actually increased symptoms that prompted some of these young patients to contemplate suicide as a way of dealing with their depression. 
Over-reporting is a problem as well. In a phenomenon known as ‘salami slicing’, researchers take the results from a single trial (the salami) and slice the results into several reports without making clear that the individual reports are not independent studies. In this way, a single ‘positive’ trial can appear in several journals in different articles, thereby introducing a bias. 
Here again, registering trials at inception with unique identifiers for every study will help to reduce the confusion that can result from this practice.
Next: What can happen if all the relevant, reliable evidence is not assessed?