Kunz R, Oxman A D
Department of Nephrology, Charité, Berlin, Germany.
BMJ. 1998 Oct 31;317(7167):1185-90. doi: 10.1136/bmj.317.7167.1185.
To summarise comparisons of randomised clinical trials and non-randomised clinical trials, trials with adequately concealed random allocation versus inadequately concealed random allocation, and high quality trials versus low quality trials where the effect of randomisation could not be separated from the effects of other methodological manoeuvres.
Systematic review.
Cohorts or meta-analyses of clinical trials that included an empirical assessment of the relation between randomisation and estimates of effect.
Cochrane Review Methodology Database, Medline, SciSearch, bibliographies, hand searching of journals, personal communication with methodologists, and the reference lists of relevant articles.
Relation between randomisation and estimates of effect.
Eleven studies that compared randomised controlled trials with non-randomised controlled trials (eight for evaluations of the same intervention and three across different interventions), two studies that compared trials with adequately concealed random allocation and inadequately concealed random allocation, and five studies that assessed the relation between quality scores and estimates of treatment effects, were identified. Failure to use random allocation and concealment of allocation were associated with relative increases in estimates of effects of 150% or more, relative decreases of up to 90%, inversion of the estimated effect and, in some cases, no difference. On average, failure to use randomisation or adequate concealment of allocation resulted in larger estimates of effect due to a poorer prognosis in non-randomly selected control groups compared with randomly selected control groups.
Failure to use adequately concealed random allocation can distort the apparent effects of care in either direction, causing the effects to seem either larger or smaller than they really are. The size of these distortions can be as large as or larger than the size of the effects that are to be detected.
总结随机对照试验与非随机对照试验、随机分配隐藏充分的试验与随机分配隐藏不充分的试验，以及随机化效果无法与其他方法学操作效果相区分的高质量试验与低质量试验之间的比较。
系统评价。
包括对随机化与效应估计之间关系进行实证评估的临床试验队列研究或荟萃分析。
Cochrane系统评价方法学数据库、Medline、SciSearch、参考文献目录、手工检索期刊、与方法学家的个人交流以及相关文章的参考文献列表。
随机化与效应估计之间的关系。
共纳入11项比较随机对照试验与非随机对照试验的研究（8项针对同一干预措施的评价，3项针对不同干预措施的评价）、2项比较随机分配隐藏充分的试验与随机分配隐藏不充分的试验，以及5项评估质量评分与治疗效果估计之间关系的研究。未采用随机分配和分配隐藏与效应估计相对增加150%或更多、相对减少高达90%、估计效应反转以及在某些情况下无差异相关。平均而言，未采用随机化或充分的分配隐藏会导致效应估计值更大，这是因为与随机选择的对照组相比，非随机选择的对照组预后较差。
未采用充分隐藏的随机分配可能会在两个方向上扭曲护理的表观效果，导致效果看起来比实际更大或更小。这些扭曲的大小可能与要检测的效果大小一样大或更大。
