Abstract
Sixty-four subjects, aged between 15 and 72 years were told how to carry out a problem-solving task, whose difficulty could be varied by changing the position of an index card in relation to a display and control. The outstanding feature, particularly of the older group's performance was the repetition of the same two kinds of mistake. Their failure seemed mainly due to an inability to rid themselves of their wrong “solutions,” in spite of the constant information indicating the mistake at each position. Whilst these differences between the age groups were hot pronounced at the easiest task, they were at the most difficult. It is suggested that: Firstly, the procedure of giving a subject the solution to a problem and observing how he reconciles his information with it, though only operating in a circumscribed area of problem-solving, is a genuine example of thinking, and, particularly where certain kinds of mistake can be predicted, it enables the experimenter to gain a more exact impression of what a subject is doing. Secondly, the manipulation of spatial contiguities lends itself to flexible and fairly precise measurement, and the principle might usefully be extended to other variables such as size, time intervals, colour, etc.

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