If the substituted picture has
the average similarity value of 80 per cent and we make the experiment
with 10 persons, the substitution may be discovered by 7 persons and
remain unnoticed by 3. We can now easily vary every one of the factors
involved. If instead of 6 cards, we take 10, it may be that only 4 out
of 10 persons, instead of 7, will discover the substitution, while if
we take 4 cards instead of 6, perhaps 9 persons out of 10 will
recognize the difference under these otherwise equal conditions. Only
an especially careless observer will overlook it. But instead of
changing the number of objects, we may change the periods of exposure.
If we show the 6 cards only for 2 seconds instead of 5 seconds, the
number of those who recognize the difference may sink from 7 to 5 or
4, and if we make the time considerably longer, we shall of course
reach a point where all 10 will recognize the substitution. The same
holds true of the shortening or lengthening of the time-interval
between the two presentations. The third variable factor is the
similarity itself. If instead of one church, not another church, but a
theatre or a skyscraper is shown, that is, if the similarity value of
80 per cent sinks down to a similarity of 60 per cent or 50 per cent,
the number of those who recognize the substitution will again become
larger; if, on the other hand, the substituted card shows the same
church, only from a slightly different angle, bringing the similarity
value up to 90 per cent or 95 per cent, the number of observers who
recognize the substitution may sink to 2 or 3.
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