The perusal of the reference lists of articles is widely used to identify additional articles that may be relevant. The problem with this approach is that the act of citing previous work is far from objective and retrieving literature by scanning reference lists may thus produce a biased sample of studies. There are many possible motivations for citing an article, ranging from decoration to showing up-to-dateness and knowledge. Brooks88 interviewed academic authors from various faculties at the University of Iowa and asked for the reasons for citing each reference in one of the authors' recent articles. Persuasiveness, the desire to convince peers and substantiate their own point of view emerged as the most important reason for citing articles. Brooks concluded that authors advocate their own opinions and use the literature to justify their point of view: "Authors can be pictured as intellectual partisans of their own opinions, scouring the literature for justification".88 In G0tzsche's analysis82 of trials of non-steroidal antiinflammatory drugs in rheumatoid arthritis, trials demonstrating a superior effect of the new drug were more likely to be cited than trials with negative results. Similarly, trials of cholesterol lowering to prevent coronary heart disease were cited almost six times more often if they were supportive of cholesterol lowering (see also Box 3.5).89 Overcitation of unsupportive studies can also occur. Hutchinson et al.90 examined reviews of the effectiveness of pneumococcal vaccines and found that unsupportive trials were more likely to be cited than trials showing that vaccines worked.
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