But while we may choose to pause and speculate as to what confounding variables may
appear, scientific method does not obligate us to do so. We know that confounding
variables are possible in non-double-blind experiments, and the number that we are able
to imagine is limited only by the time that we allocate to trying. If I cared to spend
a few hours thinking about it, I could write several pages of possibilities. If I chose
to spend a few months thinking about it, I could write a book of possibilities. I am
able to imagine confounding variables either improving health or impairing it at the low
end of the alcohol-consumption continuum, and as well either improving or impairing
health at the high end of the alcohol-consumption continuum. Scientific method does not
require us to know for certain what and how many confounding variables may appear to
destroy the validity of an experiment which is not double-blind; rather, scientific
method assures us that it is so likely that one or more confounding variables will make
their appearance in a non-double-blind experiment, that such an experiment must be
considered to be fatally defective, and that no cause-effect conclusion can ever be
drawn from it with confidence.
Thus, no valid experiment exists. In short, we can be sure that no experiment has ever
been conducted to ascertain the effect of long-term alcohol consumption on longevity,
and that if such an experiment had ever been conducted, the impossibility of its being
double-blind, or even blind, would render it inconclusive.
The French Paradox Research
Must Have Been Correlational
But if the data featured in your 60 Minutes broadcast was not experimental, then what
was it? It must, by default, have been correlational. That is, rather than subjects
being assigned randomly to groups and being required to drink a given volume of alcohol
each day, it must have been merely observed what volume of alcohol they chose to drink
each day.
Alcohol consumption would be measured by self-report. Well, it is not quite true that
the experimenter would observe what volume of alcohol his subjects drank daily. It
would be impractical to follow subjects around and actually see how much alcohol they
consumed in restaurants, in bars, in their homes. Much more likely is that every once
in a long while, the subjects would be mailed a questionnaire asking them to report how
much alcohol they had been drinking lately. The inability to measure alcohol
consumption directly is already a weakness - subjects might not remember accurately how
much they had been drinking, or they might experience some pressure to distort how much
they had been drinking either upward or downward. However, this is not at all the big
weakness that I want to bring out, so let us get to that without further delay.
We have already seen that random assignment guarantees pre-treatment equality on all
dimensions. I first recapitulate that in the case of the random assignment of subjects
to groups in an experiment, we were guaranteed that the subjects in each group would be
initially equivalent on every conceivable dimension. The larger the random groups, the
closer to being precisely equal on every conceivable dimension would they become. Thus,
in a properly designed and executed double-blind experiment, any differences that
subsequently arose between groups would have to be attributed to the different
treatments that the experiment had administered to them - for example, if some groups
lived longer than others, nothing else would be able to explain this except that some
groups had consumed a different volume of wine than others.
Natural assignment guarantees pre-treatment inequality on many dimensions. But in a
correlational study, subjects are not assigned to groups randomly, they assign
themselves to groups naturally. A subject who is in a no-wine group, for example, is