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Re: Cases and Controls
John & Group,
Thanks for the tip of Rothman/Greenland's tome on Modern Epidemiology. I
can't say it's interesting reading, but it is much more current than the
epidemiology text I have on my shelf.
The multivariate regression studies "averages within levels of multiple
regressors". I take this to mean that smoking is not corrected on the
individual level, but at the group level by some pre-established average.
The section on population controls says that "we would employ density
sampling, in which an individual's control selection probability is
proprotional to the individual's person-time risk." This (at least to me)
supports the premise that smoking needs to be more adequately controlled,
especially since smoking and radon, at least in part, have the same
underlying risk, but the controls and cases have very different
person-times. Let me explain,
The purported risk to individuals from radon is due to alpha radiation,
particularly from radon progeny. One of the purported risks to smokers (and
those exposed to second hand smoke) is alpha radiation from Po-210 (also a
radon progeny). Smoking tends to expose smokers to far greater doses of
alpha radiation that radon. In case-control studies where smoking
prevalence is greatly different between the case and control subjects, the
two groups cannot be adequately compared because the most significant
contribution of alpha radiation (i.e. smoking) is not controlled.
If multivariate regressions adequately corrected for smoking, then radon
studies including smokers with appropriate corrections should agree with
studies that exclude smokers. Generally, they do not. Studies with smokers
tend to have a small positive correlation for radon, whereas studies
excluding smokers have been negative. That makes me concerned that
multivariate regressions being applied to data sets do not adequately
correct for smoking.
To remedy the situation, smoking habits between cases and controls should
be the same. Controls represent the general population so that the
percentage of smokers should be consistent with the group. Reducing the
percentage of smokers in the case group to match the controls would lead to
selection bias. The best options is to exclude smoking altogether in both
case and control groups.
Advantages of excluding smokers in radon case-control studies:
(1) The cases and controls can be more closely matched.
(2) No statistical correction needs to be applied for smoking.
(3) Confounding due to chemical carcinogens present in cigarettes is
eliminated.
(4) Since smoking reduces radon levels, radon levels will be higher and
more compatible with most of the general population, considering that most
people do not smoke.
(5) Investigators have reported that Retrospective (track-etch on glass)
Radon Detectors are more variable in residences of smokers than
non-smokers. These results would be more consistent.
(6) Investigators can include in their research other low order potential
carcinogins, such as prior lung diseases, second hand smoke, occupations
exposures, etc. The focus of the studies can be "What causes lung cancer in
the 10% of men and 15% of women who do not smoke?"
Disdvantages of excluding smokers in radon case-control studies:
(1) The number of lung cancer cases from which to study falls off
precipitously when smokers are excluded. Regardless, the numbers should be
similar to other less prevalent cancers and sufficient information should
be attainable to obtain meaningful conclusions.
(2) The percentage of smokers in the control group would not be the same as
the general population. This is acceptable since about 75% of the general
population does not smoke.
Sorry for the long diatribe.
Tom
John Williams wrote:
>
> Tom,
>
> Adjustment for confounding is a fairly straightforward procedure in
> Epidemiology. See Rothman's and Greenland's Modern Epidemiology text
> for more details. There is no algorithm, but rather part of the
> multivariate analysis. I just looked over the Iowa AJE paper and
> they did check for residual confounding by smoking and did not detect
> any.
>
> http://www.amazon.com/exec/obidos/ASIN/0316757802/ref=pd_sim_books/103
> -0905965-4263056
>
> Sent by Law Mail
--
Thomas Mohaupt, M.S., CHP
University Radiation Safety Officer
104 Health Sciences Bldg
Wright State University
Dayton, Ohio 45435
tom.mohaupt@wright.edu
(937) 775-2169
(937) 775-3761 (fax)
"An investment in knowledge gains the best interest." Ben Franklin
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