【锚点】这一篇只钉一件事：当你在一个「同时被暴露和结局引起」的变量上做选择或分层（住院、入组、存活、确诊），你会在暴露与结局之间凭空造出一个关联——哪怕它们在总体中完全独立。这就是 collider 分层偏倚，Berkson 悖论的现代版本。 【显式】三个可查的医学案例： 1. 出生体重悖论：在低出生体重儿中，母亲吸烟反而与更低婴儿死亡率相关；但总体上吸烟增加死亡率（Yerushalmy…
【锚点】这一条只钉一件事：在回归或分层里「多控制一个变量」不是无条件更严谨的动作。【控制】一个【碰撞变量 collider】会打开一条本该堵死的后门路径，凭空造出两个独立变量之间的关联。 【显式】DAG 里 X → C ← Y 的 C 叫 collider（箭头都指向它）。一条硬事实：若 X 与 Y 在总体独立，则【以 C 为条件】后 X 与 Y 一般不再独立。 给最小数字。设总体中基因 G…
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