Re: Cancer Assessment Press Release

["John Moulder" <jmoulder@post.its.mcw.edu>]

> So what this implies (playing the devil's advocate) is that the 
> nuclear industry is the ONLY industry that has this physical screening and
> regular periodic exams to maintain worker health.  Also, we have had supreme
> luck in selecting individuals to work in this industry that natually have 
> less incidence of cancer than the rest of the population.

The above argument assumes that the 'healthy worker effect" is seen only in 
the nuclear industry.  This effect is seen in all sorts of industries for all 
sorts of diseases.  For a recent review article on the subject, see below.

> More curiosity questions for the epidemiologists out there.  Does 
> the "general population" in these studies include those below or 
> well below the poverty line?

In general it does, although adjustments are often made for socioeconomic 
status.

> How do you deal with the confounding factor (is that the correct 
> term?) of people who refuse to take care of themselves or refuse 
> medical treatment on religious grounds?

In general, when looking at potential occupational carcinogens you do not 
compare with the "general population", you compare to workers in the same 
industry, with similar salaries, who do not have this exposure.

You also look for exposure-response relationships within the exposed cohort.

> Trying to do a balanced and 
> fair epidemiological study sounds liek a complete nightmare.  Seems 
> like there's way too much room for 
> bias and opinion to seep in.

Yes to all of the above.  That's why epidemiology is such a poor tool for 
detecting weak risks (that is, relative risks less than about 3), particularly 
when the exposed population is small.
  

Arrighi HM and Hertz-Picciotto: The evolving concept of the healthy worker 
survivor effect.  Epidemiology.  5(2):189-96, 1994

Abstract:
The "healthy worker survivor effect" describes a continuing selection process 
such that those who remain employed tend to be healthier than those who leave 
employment. In an analysis of exposure-response patterns in an occupational 
study, the healthy worker survivor effect generally attenuates an adverse 
effect of exposure. In practical terms, such attenuation will be more 
problematic when evaluating subtle rather than strong associations. The use of 
an internal referent does not guarantee elimination of this effect, since by 
definition, it manifests within an occupational cohort. Although documented 
over 100 years ago, there is little consensus regarding the most appropriate 
method to control for the healthy worker survivor effect. Four methods have 
been proposed for its control: (1) restriction of the cohort to survivors of a 
fixed number of years of follow-up, (2) lagging the exposure to exclude recent 
exposure incurred by those who remained on the job, (3) adjusting for 
employment status as a confounder, and (4) treating the healthy worker 
survivor effect simultaneously as an intermediate and confounding variable by 
means of the G-null test or its extension, G-estimation analysis, using 
structurally nested failure time models. This paper reviews the concept of the 
healthy worker survivor effect and the four methods to control for it. 


John Moulder (jmoulder@its.mcw.edu)