Odgaard-Jensen Jan, Vist Gunn E, Timmer Antje, Kunz Regina, Akl Elie A, Schünemann Holger, Briel Matthias, Nordmann Alain J, Pregno Silvia, Oxman Andrew D
Norwegian Knowledge Centre for the Health Services, PO Box 7004, St. Olavs Plass, Oslo, Norway, N-0130.
Cochrane Database Syst Rev. 2011 Apr 13;2011(4):MR000012. doi: 10.1002/14651858.MR000012.pub3.
Randomised trials use the play of chance to assign participants to comparison groups. The unpredictability of the process, if not subverted, should prevent systematic differences between comparison groups (selection bias). Differences due to chance will still occur and these are minimised by randomising a sufficiently large number of people.
To assess the effects of randomisation and concealment of allocation on the results of healthcare studies.
We searched the Cochrane Methodology Register, MEDLINE, SciSearch and reference lists up to September 2009. In addition, we screened articles citing included studies (ISI Science Citation Index) and papers related to included studies (PubMed).
Eligible study designs were cohorts of studies, systematic reviews or meta-analyses of healthcare interventions that compared random allocation versus non-random allocation or adequate versus inadequate/unclear concealment of allocation in randomised trials. Outcomes of interest were the magnitude and direction of estimates of effect and imbalances in prognostic factors.
We retrieved and assessed studies that appeared to meet the inclusion criteria independently. At least two review authors independently appraised methodological quality and extracted information. We prepared tabular summaries of the results for each comparison and assessed the results across studies qualitatively to identify common trends or discrepancies.
A total of 18 studies (systematic reviews or meta-analyses) met our inclusion criteria. Ten compared random allocation versus non-random allocation and nine compared adequate versus inadequate or unclear concealment of allocation within controlled trials. All studies were at high risk of bias.For the comparison of randomised versus non-randomised studies, four comparisons yielded inconclusive results (differed between outcomes or different modes of analysis); three comparisons showed similar results for random and non-random allocation; two comparisons had larger estimates of effect in non-randomised studies than in randomised trials; and two comparisons had larger estimates of effect in randomised than in non-randomised studies.Five studies found larger estimates of effect in trials with inadequate concealment of allocation than in trials with adequate concealment. The four other studies did not find statistically significant differences.
AUTHORS' CONCLUSIONS: The results of randomised and non-randomised studies sometimes differed. In some instances non-randomised studies yielded larger estimates of effect and in other instances randomised trials yielded larger estimates of effect. The results of controlled trials with adequate and inadequate/unclear concealment of allocation sometimes differed. When differences occurred, most often trials with inadequate or unclear allocation concealment yielded larger estimates of effects relative to controlled trials with adequate allocation concealment. However, it is not generally possible to predict the magnitude, or even the direction, of possible selection biases and consequent distortions of treatment effects from studies with non-random allocation or controlled trials with inadequate or unclear allocation concealment.
随机试验利用随机化过程将参与者分配到各个比较组。如果该过程未被破坏，其不可预测性应能防止比较组之间出现系统性差异（选择偏倚）。因随机因素导致的差异仍会出现，通过对足够多的人进行随机化可将这些差异降至最低。
评估随机化及分配方案隐藏对医疗保健研究结果的影响。
我们检索了Cochrane方法学注册库、MEDLINE、SciSearch以及截至2009年9月的参考文献列表。此外，我们筛选了引用纳入研究的文章（ISI科学引文索引）以及与纳入研究相关的论文（PubMed）。
符合条件的研究设计包括队列研究、医疗保健干预措施的系统评价或荟萃分析，这些研究比较了随机分配与非随机分配，或随机试验中分配方案隐藏充分与不充分/不明确的情况。感兴趣的结局包括效应估计值的大小和方向以及预后因素的不均衡性。
我们独立检索并评估了似乎符合纳入标准的研究。至少两名综述作者独立评估方法学质量并提取信息。我们为每个比较准备了结果的表格总结，并对各项研究的结果进行定性评估，以确定共同趋势或差异。
共有１８项研究（系统评价或荟萃分析）符合我们的纳入标准。其中１０项比较了随机分配与非随机分配，９项比较了对照试验中分配方案隐藏充分与不充分或不明确的情况。所有研究都存在较高的偏倚风险。对于随机研究与非随机研究的比较，四项比较结果不明确（不同结局或不同分析模式之间存在差异）；三项比较显示随机分配与非随机分配结果相似；两项比较显示非随机研究中的效应估计值大于随机试验；两项比较显示随机研究中的效应估计值大于非随机研究。五项研究发现分配方案隐藏不充分的试验中的效应估计值大于分配方案隐藏充分的试验。其他四项研究未发现统计学上的显著差异。
随机研究与非随机研究的结果有时存在差异。在某些情况下，非随机研究的效应估计值更大，而在其他情况下，随机试验的效应估计值更大。分配方案隐藏充分与不充分/不明确的对照试验结果有时也存在差异。当出现差异时，大多数情况下，分配方案隐藏不充分或不明确的试验相对于分配方案隐藏充分的对照试验，效应估计值更大。然而，通常无法预测非随机分配研究或分配方案隐藏不充分或不明确的对照试验中可能存在的选择偏倚的大小，甚至方向，以及由此导致的治疗效果扭曲。
