背景：系统评价（SRs）是有关医疗保健干预的重要信息来源。进行良好的SR的关键组成部分是全面的文献检索。关于非英文报告的贡献的证据有限，未发表的研究,论文及其对荟萃分析结果的影响。
方法：我们的样本包括来自三个Cochrane审查组的SR：急性呼吸道感染（ARI），传染病（ID），发展中的心理社会和学习问题（DPLP）（n=129）。结果包括：1）搜索并包括每种研究类型的评论的比例；2）每种研究类型代表的相关研究的比例；3）对每种研究类型的主要荟萃分析的结果和结论的影响。
结果：大多数SR搜索非英语研究；然而，这些研究仅纳入12%的综述,占纳入研究的不到5%.仅有4篇综述的结果发生了变化（总样本=129）；在两种情况下，这种变化对结果的统计或临床意义没有影响。大多数SR搜索未发表的研究，但大多数不包括这些（只有6％），它们占纳入研究的2％。在大多数情况下，纳入未发表的研究的影响很小；在一个仅依赖于未发表的数据的案例中观察到了重大影响。ARI（9％）和ID（3％）中很少有评论搜索论文，而DPLP中只有65％。总的来说,论文仅包含在9项SR中，占纳入研究的不到2％。在大多数情况下，结果的变化可以忽略不计或很小；在注意到较大变化的情况下，没有论文的估计比较保守。
结论：大多数SR搜索非英语和未发表的研究；然而，这些研究只占纳入研究的一小部分,很少影响综述的结果和结论.纳入这些研究类型可能会在相关研究很少的情况下产生影响，或者在出版的文献中有可疑的既得利益。我们发现SRs是否搜索论文存在很大差异；在大多数包括论文的评论中，这些对结果影响不大。
BACKGROUND: Systematic reviews (SRs) are an important source of information about healthcare interventions. A key component of a well-conducted SR is a comprehensive literature search. There is limited evidence on the contribution of non-English
reports, unpublished studies, and dissertations and their impact on results of meta-analyses.
METHODS: Our sample included SRs from three Cochrane Review Groups: Acute Respiratory Infections (ARI), Infectious Diseases (ID), Developmental Psychosocial and Learning Problems (DPLP) (n = 129). Outcomes included: 1) proportion of reviews that searched for and included each study type; 2) proportion of relevant studies represented by each study type; and 3) impact on results and conclusions of the primary meta-analysis for each study type.
RESULTS: Most SRs searched for non-English studies; however, these were included in only 12% of reviews and represented less than 5% of included studies. There was a change in results in only four reviews (total sample = 129); in two cases the change did not have an impact on the statistical or clinical significance of results. Most SRs searched for unpublished studies but the majority did not include these (only 6%) and they represented 2% of included studies. In most cases the impact of including unpublished studies was small; a substantial impact was observed in one
case that relied solely on unpublished data. Few reviews in ARI (9%) and ID (3%) searched for dissertations compared to 65% in DPLP. Overall, dissertations were included in only nine SRs and represented less than 2% of included studies. In the majority of cases the change in results was negligible or small; in the
case where a large change was noted, the estimate was more conservative without dissertations.
CONCLUSIONS: The majority of SRs searched for non-English and unpublished studies; however, these represented a small proportion of included studies and rarely impacted the results and conclusions of the review. Inclusion of these study types may have an impact in situations where there are few relevant studies, or where there are questionable vested interests in the published literature. We found substantial variation in whether SRs searched for dissertations; in most reviews that included dissertations, these had little impact on results.
