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Re: Cases and Controls
Dear Tom and LDR followers,
Thank you for the suggested mechanism that the alpha radiation in smoke is its carcinogen. I had assummed it was only the chemically carcinogenic
tars.
I do not understand how radon in non-smokers homes would be increased by excluding smokers from the study
Controls as identical to cases as possible is so essential to validity, it is the very reason for double-blind, "gold standard" experiments with
placebo!
"Adjusting" statistically, as by extending a trend beyond the data points, is what created the LNT mess in the first place!
We need an experiment, with true controls (randomized to placebo, double blind) like Cameron's package under mattress proposal.
Howard Long
Tom Mohaupt wrote:
> 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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