Re: Confounders and Coincidences

[epirad@mchsi.com]

Dear Otto,



Yes, using ecologic data to control confounding is helpful sometimes.  In 

fact, it does help especially if there are not large inter and intra county 

non linear effects.  However, we know without question that these non linear 

sources of confounding and effect modification exist in Dr. Cohen's data.  



Two papers really do a nice job of adressing this problem.



Ecological bias, confounding, and effect modification 



S Greenland and H Morgenstern 



Division of Epidemiology, UCLA School of Public Health 90024.

 

Ecological bias is sometimes attributed to confounding by the group variable 

(ie the variable used to define the ecological groups), or to risk factors 

associated with the group variable. We show that the group variable need not 

be a confounder (in the strict epidemiological sense) for ecological bias to 

occur: effect modification can lead to profound ecological bias, whether or 

not the group variable or the effect modifier are independent risk factors. 

Furthermore, an extraneous risk factor need not be associated with the study 

variable at the individual level in order to produce ecological bias. Thus the 

conditions for the production of ecological bias by a covariate are much 

broader than the conditions for the production of individual-level confounding 

by a covariate. We also show that standardization or ecological control of 

variables responsible for ecological bias are generally insufficient to remove 

such bias. 



--------------------------------------

Ecologic versus individual-level sources of bias in ecologic estimates of 

contextual health effects 



Sander Greenland 



Department of Epidemiology, UCLA School of Public Health, and Department of 

Statistics, UCLA College of Letters and Science, 22333 Swenson Drive, Topanga, 

CA 90290, USA. 

 

Bill's insert - (Does this first sentence sound familiar?)



Abstract:



A number of authors have attempted to defend ecologic (aggregate) studies by 

claiming that the goal of those studies is estimation of ecologic (contextual 

or group-level) effects rather than individual-level effects. Critics of these 

attempts point out that ecologic effect estimates are inevitably used as 

estimates of individual effects, despite disclaimers. A more subtle problem is 

that ecologic variation in the distribution of individual effects can bias 

ecologic estimates of contextual effects. The conditions leading to this bias 

are plausible and perhaps even common in studies of ecosocial factors and 

health outcomes because social context is not randomized across typical 

analysis units (administrative regions). By definition, ecologic data contain 

only marginal observations on the joint distribution of individually defined 

confounders and outcomes, and so identify neither contextual nor individual-

level effects. While ecologic studies can still be useful given appropriate 

caveats, their problems are better addressed by multilevel study designs, 

which obtain and use individual as well as group-level data. Nonetheless, such 

studies often share certain special problems with ecologic studies, including 

problems due to inappropriate aggregation and problems due to temporal changes 

in covariate distributions. 



Regards, Bill

bill-field@uiowa.edu

> At 03:52 PM 5/8/03 +0000, Bill Field wrote:

> >Dr. Cohen wrote - 

> >

> >Unfortunately, ecological control of a covariate contributing to ecologic

> bias 

> >is generally inadequate to remove the bias produced by the covariate even in 

> >the absence of measurement error. 

> ****************************************************

> May 8, 2003

> 

> Dear Bill:

> 

> Thanks for your comment. You bring up an important point. 

> 

> But it seems to me that the fact that ecological control may not remove the

> bias does not mean that it never helps. If all you have is ecological data,

> isn't testing the effect of ecological control a useful exercise?

> 

> Otto

> 

> **********************************************

> Prof. Otto G. Raabe, Ph.D., CHP

> Center for Health & the Environment

> (Street Address: Bldg. 3792, Old Davis Road) 

> University of California, Davis, CA 95616

> E-Mail: ograabe@ucdavis.edu

> Phone: (530) 752-7754   FAX: (530) 758-6140

> ***********************************************



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