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Now we have this thing
called the healthy worker effect, which is a
famous kind of selection bias. That's when
we're selecting individuals from the work
world, but people who work are not the same
as people who don't work. If fact people who
work are typically healthier than the rest
of the population. This is a real problem,
because if you think about where most of our
mortality data comes from, it comes from working
populations, as well we see something resembling
the healthy donor effect when we look at blood
donation. So people who donate blood are typically
healthier than people who don't donate blood
and a lot of our data about the prevalence
of disease comes from analyzing donated blood.
This is pretty important in mortality studies,
the healthy worker effect is, because again,
people who work are healthier than those who
don't work. And if we are comparing mortality
rates in different jobs to the general population,
it always seems as if those majority rates
aren't as bad as we think, but the general
population may not be as healthy as those
who work, so this is an underestimate perhaps,
or an overestimate, depending upon your perspective
of the effect of mortality.