Re: Teaching physics to biology students

In article <1142976497.146304.306410@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Edward Green <spamspamspam3@xxxxxxxxxxx> wrote:
Gregory L. Hansen wrote:

In article <1142950293.575497.4400@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Edward Green <spamspamspam3@xxxxxxxxxxx> wrote:
Gregory L. Hansen wrote:


Another point, mentioned about medical research which is in the same
vein as routine data runs in physics: am I right in thinking that a
significant proportion of medical research is simply statistical
investigation? A carefully controlled statistical experiment may point
to the existence of an undisclosed mechanism. A loosely controlled one
may merely be suggestive, or may do more harm than good -- since the
"suggestion" will be taken as evidence by people prejudiced towards the
hypothesis.

In some sense, *every* experiment is a statistical investigation! Even
simple physical measurements must be reported with an error bar.

True. I was thinking, as Ken suggested, of experiments which might
test if there were any correlation at all.

Okay, we can dispense with vague statements that exclude nothing, then.


Now I think we might want to distinguish between experiments ... ahem,
"studies" ... which merely sought causation, and those which sought
effect. What's the difference? I suggest the difference may be that
in a statistical study looking for an "effect", steps have been taking
to reasonably randomize all uncontrolled sources of variation across
the treated and untreated populations. Since we can never know all
sources of variation, this can only be done by randomized assignment to
treated and untreated sub-populations. This cannot be done given a
pre-existing (self-treated) population. Yet I think this latter
situation often occurs in published studies, taking as evidence that
such studies frequently see the light of day through third party
popular publication.

Causation versus correlation is more complicated than that. The classical
pedagogical example is that the number of drownings in a population
increases with the amount of ice cream eaten. It might not occur to the
student, at first, that one doesn't cause the other; rather, both are
driven by warmer weather in the summer.

Following my inclination to connect to actual research rather than
sticking with hypotheticals, enough studies have shown a correlation with
body fat and early mortality. Does that mean body fat causes it? We
could compare health indicators of people before and after losing weight,
except that losing weight involves changes in diet and exercise, which has
health benefits even if no weight is lost. A convincing test would have
to avoid such things. The liposuction study showed that simply sucking
out the subcutaneous layer doesn't change the health indicators. There's
reason to think that the visceral fat that's wrapped around internal
organs has a different health effect than subcutaneous fat, so it could be
that the wrong fat was studied. Maybe somebody will be able to test that
directly by figuring out a way to suck out that stuff. Otherwise how can
that even be tested, except through the filters of theory?

An interesting sub-population in one study was former athletes, in high
school or college, who'd let themselves go. They had a higher mortality
rate in later years than more sedentary people with a more stable body
weight did. Now, it's a little-known fact that when Russian athletes
retire, they're put on a lengthy de-training program that gradually
decreases their exercise intensity, the purpose of which is to avoid
health problems like perforations of heart valves that can occur if a
professional athlete suddenly stops training. The Russians are somewhat
ahead of us in sport science. That little-known fact suggests the
hypothesis that the increased mortality has to do with the cessation of
training rather than the accumulation of fat. If we hurry up and push a
grant proposal through, we ought to be able to get a definitive test of
that hypothesis in about twenty years.

....

What I was suggesting, however, is that there are a significant
proportion of "type I" investigations out there in the medical field.
I did not define "significant", so I said almost nothing. ;-) However,
I'm going to go out on a limb here. I estimate that at least 50% of
published medical studies which are subsequently picked up by the
popular media for sound-byte status are type-I statistical
investigations.

That is, with our refined classification, what I meant to suggest by my
off-hand remark.


Hard to say if we just go by what's filtered through the media. They
don't report things like error bars, they just report things like "NEW
DISCOVERY!" Dr. "Squat" Hatfield, a sports psychologist, has followed the
headlines resulting from a particular study that showed the health
benefits of regular exercise, with higher intensity giving more benefits,
although even low levels of exercise gave some benefit. After a few
cycles through the popular media they were reporting that you don't have
to exercise hard to get the benefits. They seemed to forget the part that
if you don't exercise hard you won't get much benefit, and that getting
great benefits is still going to be hard work. A measure of effect versus
intensity had been turned into yes/no.


--
"A good plan executed right now is far better than a perfect plan
executed next week."
-Gen. George S. Patton
.

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