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Response from Dr. Field concerning the recent postings
Concerning the numerous posts about the Iowa Radon Lung Cancer Case-Control
Study and Dr. Cohen's ecologic studies, Dr. Field asked me to post this
general response.
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The main argument against Dr. Cohen's studies (see also our Forum article
and follow up letters in the Health Physics Journal concerning this topic)
is and always has been this: aggregation in an ecologic study can lead to
biased estimates. The bias can be so bad as to yield estimates with the
wrong sign, as Dr. Jay Lubin has shown. Increasing the sample size does not
reduce the bias in any way; it simply leads to more precise estimates. In
other words, you would be more precisely estimating the wrong thing.
Whether our findings agree with the hypotheses generated in Cohen's ecologic
study is irrelevant. Ecologic studies do not have the ability to assess
risk. I am intimately aware of how the data was collected and analyzed in
both the Iowa Radon Lung Cancer Case-Control Study and Dr. Cohen's ecologic
studies. I feel the results from Cohen's ecologic studies are dubious
because of the increased risk of biases inherent in his study design.
Larger sample sizes cannot overcome bias (a clear case of quality versus
quantity).
As far as the questions concerning matching cases and controls in a
case-control study by smoking status -
In speaking with one of the statisticians on the Iowa study, he felt as I do
that there is no practical way to match cases and controls so as to adjust
for the effects of smoking on lung cancer risk. Smoking is so strongly and
intricately associated with lung cancer that one would have to match on a
multitude of factors, including intensity, duration, and time since
cessation. Furthermore, there would undoubtedly be other covariates such as
age, education, family history, and occupational exposures that would not be
matched and would have to be controlled for in some fashion; i.e. multiple
logistic regression.
The methods we used were standard methods to control for smoking. In the
Iowa Residential Radon Case-Control Study, multiple logistic regression was
used to model the effect of residential radon exposure on lung cancer risk.
Included in the regression model were variables to adjust for the effects of
smoking. Specifically, the model included continuous variables for the
length of time that individuals smoked, the number of cigarettes smoked
during that time, and time since smoking cessation (for ex-smokers).
Quoting Hosmer and Lemeshow's authoritative book, "Applied Logistic
Regression":
"One generally considers a multivariate analysis for a more comprehensive
modeling of the data. One goal of such an analysis is to statistically
adjust the estimated effects of each variable in the model for differences
in the DISTRIBUTION of and associations among the other independent
variables. Applying this concept to a multivariate logistic regression
model, we may surmise that each estimated coefficient provides an estimate
of the log odds [of lung cancer] adjusting for all other variables [smoking]
included in the model."
It may be of interest to know that an independent analysis of the Iowa Radon
Case-Control Radon Study will be published within the next year or so as
part of the National Residential Pooling paper.
I know Dr. Cohen and some others will not agree with some of the above
statements. That is fine. It is rare that there are topics we all do agree
on. I think disagreement is a healthy part of the scientific process. As
health professionals, I am sure most of us are in agreement that we should
continue to strive to reduce cancers, and in particular the leading cause of
cancer death (lung cancer) in both men and women. A good place to start
would be to actively promote efforts for smoking cessation. I am happy to
say that just yesterday we were able to get a smoke-free ordinance passed
that prohibits smoking in family restaurants here in Iowa City, IA. We are
a bit behind California in that regard, but nonetheless finally on our way.
It is my understanding that the owner of the list asked that this topic be
concluded for now on the list. Therefore, I would be happy as always to
discuss radon epidemiology by private email, please
mailto:bill-field@uiowa.edu.
Regards, Bill Field
College of Public Health
Department of Epidemiology
N222 Oakdale Hall
University of Iowa
Iowa City, Iowa 52245
319-335-4413 (phone)
Community of Science
http://expertise.cos.com/cgi-bin/exp.cgi?id=323385
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