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st: selection effect vs selection bias

From   "Michael Cha" <>
Subject   st: selection effect vs selection bias
Date   Sun, 31 Aug 2003 20:43:47 -0400

Can anyone help me with the difference between 
sample selection bias and selection effect? 

Basically, I am not sure whether they are different or not. 
If different, the following might make sense.  

As far as I know, selection effect is a problem that happens when we aggregate the micro-unit data to make the macro-unit data, while sample selection bias occurs when only small portion of specific sample are collected out of the population. In other words, not random but nonrandom sampling is the major cause for selection bias.  

Is it right? 

If not, please let me know. 

Thank you very much for your help in advance. 


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