Information Technology Reference
In-Depth Information
Steps 1 and 2 are trickier than they appear at first glance. Do the phe-
nomena under investigation depend upon the time of day as with body
temperature and the incidence of mitosis? Do they depend upon the day
of the week as with retail sales and the daily mail? Will the observations be
affected by the sex of the observer? Primates (including you) and hunters
(tigers, mountain lions, domestic cats, dogs, wolves, and so on) can readily
detect the observer's sex.
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Blocking may be mandatory because even a randomly selected sample
may not be representative of the population as a whole. For example, if a
minority comprises less than 10% of a population, then a jury of 12
persons selected at random from that population will fail to contain a
single member of that minority at least 28% of the time.
Groups to be compared may differ in other important ways even before
any intervention is applied. These baseline imbalances cannot be attributed
to the interventions, but they can interfere with and overwhelm the com-
parison of the interventions.
One good after-the-fact solution is to break the sample itself into strata
(men, women, Hispanics) and to extrapolate separately from each stratum
to the corresponding subpopulation from which the stratum is drawn.
The size of the sample we take from each block or strata need not, and in
some instances should not, reflect the block's proportion in the population.
The latter exception arises when we wish to obtain separate estimates for
each subpopulation. For example, suppose we are studying the health of
Marine recruits and wish to obtain separate estimates for male and female
Marines as well as for Marines as a group. If we want to establish the inci-
dence of a relatively rare disease, we will need to oversample female recruits
to ensure that we obtain a sufficiently large number. To obtain a rate
R
for
all
Marines, we would then take the weighted average
p
F
R
F
+
p
M
R
M
of the
separate rates for each gender, where the proportions
p
M
and
p
F
are those of
males and females in the
entire
population of Marine recruits.
FOUR GUIDELINES
In the next few sections on experimental design, we may well be preach-
ing to the choir, for which we apologize. But there is no principle of
experimental design, however obvious, and however intuitive, that
someone will not argue can be ignored in his or her special situation:
•
Physicians feel they should be allowed to select the treatment that
will best affect their patient's condition (but who is to know in
advance what this treatment is?).
The hair follicles of redheads—genuine, not dyed—are known to secrete a prostaglandin
similar to an insect pheromone.
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