Can estimation of circle size be influenced by instructions?

The circle is commonly used as a quantitative symbol in cartography but the relationship between the judged and actual size of circles is not yet dear. In the experiment reported, a free modulus magnitude estimation methodology was used to investigate the influence of instructions on estimation of circle size. One group of subjects was required to judge the size of open circles on a white background without specific instructions; a second group was required to judge the size of the circles basing their judgement on the diameter of the circles; and a third group judged the size of the circles by basing their judgement on the areas of the circles. No significant differences were found for the exponents of the power functions obtained from the three groups. The procedure resulted in power functions indicating that the size of the circles, under all conditions, were judged in proportion to their diameters yielding judgements which were all underestimates of circle area. © 1980 Taylor & Francis Group, LLC.