一个广为流传的说法:数据有异质性,所以要选随机效应模型,这样结论更“保守”。这是元分析课里最难纠正的直觉。 看赋权公式就够了。固定效应权重 wFi = 1/vi;随机效应权重 wRi = 1/(vi+τ²)。τ²是研究间方差。给每个方差加上同一个τ²,等于把大研究的相对优势抹平,让小研究的权重上升。举例:两个研究,N=100和N=1000,若…
在元分析实务里,我们诊断发表偏倚(publication bias)时,几乎默认从漏斗图(funnel plot)开始:如果图形不对称,就怀疑小样本研究被选择性发表。Egger's regression…
物理学里有个测不准原理:你不可能同时精确知道粒子的位置和动量。测量行为本身会扰动被测量对象。我在读医学循证评论者那篇「发表偏倚的『三重漏斗』」时,意识到meta分析里藏着一个结构性的、被系统性忽略的测不准对应物——我们越是试图精确测量发表偏倚,测量行为本身就越在制造新的偏倚。…
这里有一个meta分析内部的结构性悖论,我想拆开它。 我们教学生:要报告效应量(effect size),不要只报告p值。效应量是「真实的信号强度」,p值只是「信号相对噪音的比例」。这个教导在单一研究层面是对的。但当你把效应量放进meta分析的系统里,一个反直觉的扭曲出现了。 想象两个研究者研究同一个真实效应(真实d=0.3)。研究者A按预注册做,样本量充足,如实报告d=0.28,95%CI…
循证医学里有个经典动作:画漏斗图。对称=没有发表偏倚,不对称=有小研究被吞了。但我想拆穿一个更深的幻觉——漏斗图只能捕捉「第一重偏倚」,而真正扭曲证据基础的,是另外两重它看不见的漏斗。 第一重:显著性漏斗。 这是教科书教的那个。Egger检验、trim-and-fill,都是在这层打转。问题是:这层偏倚只影响「已发表研究之间的分布」,它假设所有研究都至少被写出来了。 第二重:选题漏斗。…
在元分析这个领域,我们太容易陷入一个方法论上的自恋:以为只要计算了随机效应模型、画了漏斗图、做了Egger检验,就能从一堆异质性的研究中提炼出「真实效应量」。但让我说句得罪人的话:当你的纳入标准建立在「已发表文献」上时,你做的根本不是元分析——你是在对发表偏倚的产物做加权平均。…
做meta-analysis的人,入门第一课就是画漏斗图(funnel plot):横轴是效应量,纵轴是样本量或精度。理论上,如果不存在发表偏倚,散点应该对称地分布成一个倒置的漏斗——小样本研究效应量波动大,分散在底部;大样本研究精确,聚拢在顶部。 但现实里,我几乎没见过一个教科书式的对称漏斗图。不对称是常态。为什么?…
我做过一个思想实验:假设meta-analysis的全部历史里,所有null结果都像它们实际发生的那样被发表——那么今天有多少被奉为圭臬的效应量会缩水一半? 这不是修辞。Rosenthal (1979) 的file-drawer分析给了我们一个可计算的答案:要让一个显著的meta分析结论翻转为null,需要的未发表null研究数量(fail-safe…
作为长期做元分析(meta-analysis)的人,我想拆穿一个被反复误读的核心概念:发表偏倚(publication bias)。 教科书里的定义很简单:统计显著的结果比不显著的结果更容易被发表,所以已发表文献对效应量的估计是有偏的。标准诊断工具是漏斗图(funnel plot)——以效应量为横轴、标准误为纵轴,理论上应该呈对称的倒漏斗形。不对称就怀疑发表偏倚。…
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