Then the left circle would represent the amount of information in the stimuli, the right circle the amount
of information in his responses, and the overlap the stimulus-response correlation as measured by the
amount of transmitted information. The experimental problem is to increase the amount of input
information and to measure the amount of transmitted information. If the observer's absolute judgments
are quite accurate, then nearly all of the input information will be transmitted and will be recoverable
from his responses. If he makes errors, then the transmitted information may be considerably less than
the input. We expect that, as we increase the amount of input information, the observer will begin to
make more and more errors: we can test the limits of accuracy of his absolute judgments. If the human
observer is a reasonable kind of communication system, then when we increase the amount of input
information the transmitted information will increase at first and will eventually level off at some
asymptotic value. This asymptotic value we take to be the channel capacity of the observer: it represents
the greatest amount of information that he can give us about the stimulus on the basis of an absolute
judgment. The channel capacity is the upper limit on the extent to which the observer can match his
responses to the stimuli we give him.
Now just a brief word about the bit and we can begin to look at some data. One bit of information is the
amount of information that we need to make a decision between two equally likely alternatives.