Ecological fallacy and the LNT

["Field, R. William" <bill-field@UIOWA.EDU>]

Ruth,


Ecologic fallacy is merely an inappropriate conclusion regarding

relationships at the individual level based on ecological data (Kelsey et

al. 1986).


However, ecologic studies inherently have many weaknesses that lead to an

inappropriate conclusion.    For example, 


1) in case-control studies nondifferential (or random error) exposure

misclassification usually causes bias toward the null.  However, in

an ecologic study nondifferential misclassification can drive either

sizable under or over estimation of the exposure disease relationship.



2) Using summary measures (like sales tax to represent smoking

prevalence) for a county is very inadequate to control for confounders.



3) Some factors that are not confounders at the individual level can be

at the ecologic level.  To have any hope of control for confounding

at the county level, you need very detailed information on the

confounding and exposure variable distributions within a county. 

That is why I previously suggested to Dr. Cohen that he try to obtain

this information (method suggested by Guthrie). 


4) Jim Muckerheide often cites the large numbers of counties in Dr.

Cohen's studies.  But, the availability of large numbers does not

eliminate biases no matter if you had an infinite number of

counties.  Large numbers do help improve the precision (narrow

confidence bands), but the large numbers have no effect on reducing

biases.  In other words, the finding can be precisely wrong.



5) I have seen no data from Dr. Cohen to show that the risk factors

within counties are not correlated.  Unless the risk factors are

purely additive (and I see no evidence of that), the dose response

findings for the ecologic study will be biased.


Ruth, in your example below, the population is not at all defined so the

concept of collective dose and collective risk can not be used. 

Collective dose and collective risk can only begin to be considered valid

if the exposed population can be well described and

quantified.   For example, many groups of workers who may have

a higher risk of exposure can be quantified by age, job description, sex,

length of employment and a multitude of other factors.  The general

(working) group I described above has many more demographic details than

is available in ecologic studies.  If you don't have better

demographics than are available in a county ecologic study, you likely

should not be using the collective dose concept.    



Pardon my short answer, but I am swamped with other work related demands

right now.


Best Regards, Bill




At 09:45 AM 2/12/2002 -0500, RuthWeiner@AOL.COM wrote:

> Dear
>
> RADSAFERs 
>
>
> I am working my way through both Field et al (and some ancillary papers
>
> kindly sent me by Dr. Field) and Dr. Cohen's papers (thatnk you, Bernie,
>
> for sending them).  I am trying to understand the papers with all
>
> their nuances.  I believe this can be done by a reasonably competent
>
> scientist like me (Ph. D. -- chemistry) without special training in
>
> epidemiology, but from time to time I will post questions -- real
>
> questions to which I would like real straightforward answers -- on
>
> RADSAFE.  Let me say at the outset that these questions are not
>
> intended to castigate anybody or take sides.  If I use the LNT in a
>
> question, it doesn't mean I endorse it.   The questions are for
>
> information.   (and if they sound repetitive to the individual
>
> who accused me of "me too-ism" well, that can't be
>
> helped).  So here is my first question, about the "ecological
>
> fallacy." 
>
>
> Let us say the cumulative dose to a population of 10,000 persons is 1000
>
> PERSON-rem.  Then, according to the LNT, one might expect 0.5 excess
>
> cancer in that population of 10,000 (0.0005*1000 = 0.5).  One can
>
> also, independently, say that the average dose is 0.1 rem (1000/10,000 =
>
> 0.1), or 100 mrem.  But to say that therefore the average expected
>
> cancer incidence would be 0.000005 (0.0005*0.1) is the fallacy in
>
> question.  Essentially, "average individual cancer
>
> expectation" or whatever is meaningless.  Have I got it
>
> right?  If not,what is my mistake? 
>
>
> (The LNT conversion from rem to cancer is from ICRP 90, page 22)
>
>
>
> Ruth Weiner, Ph. D. 
>
> ruthweiner@aol.com






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