Concrete Ecologic Example Myth

[epirad@mchsi.com]

Kai,



I know we have gone through discussions about the limitations of ecologic 

studies many times but there are some points that a few radsafers continue to 

miss.



You suggested the other day putting an upper limit on the error from an 

ecologic study.  Unfortuantely, this is not easy to do since the error in 

ecologic studies is unbounded.



Others problems to keep in mind - 



1) It is not possible to identify empirical sources of ecologic bias from 

aggregate data alone. Researchers must rely on prior knowledge of intergroup 

variation in the distribution of other risk factors and effect modifiers. 

We don't know this inter group variation for Dr. Cohen's data nor has he 

adjusted for it. 



2) Factors responsible for ecologic bias may not be confounders or effect 

modifiers at the individual level and that identifying the bias is even more 

difficult because factors may not even “appear” to be confounders or modifiers 

at the ecologic level.  



3) Ecologic biases can even reverse the direction of an observed association, 

especially when the range of average exposure levels across groups is small or 

the exposure under study is not a strong risk factor regardless even if you are 

using an ecologic study to test the LNT.  



4) If cross level bias is occurring (which we know it is), ecologic data can 

not be successfully used to adjust for ecologic bias.  



Many examples have been given on how Dr. Cohen's data can be biased and since 

Cohen will use his own faulty data to test the "concrete" example; providing 

a "concrete" example is doing no more than providing an explanation Cohen will 

use his own faulty data to negate.  So, the circle will never end.   

> Friends,

 

> We are all aware that ecological studies have some limitations - these issues have been discussed (some would say ad

> infinitum) on this board and elsewhere.

 

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