Errors of visual judgement in precision measurements

Abstract

Measurement is one of the most widely practiced social activities of people, and in many instances, measurements are made manually by individuals. On the other hand, automation of measurement has made rapid progress in recent years, but is not always advantageous. For example, even automatic measuring instruments are calibrated manually in many instances, and manual calibration will still be needed in the future. One of the problems to be considered in conjunction with manual measurement is error in measurement, especially personal error, which varies in degree according to the individual.

This paper is concerned with the causes of personal error in precision measurements which require strict control of errors, especially those of measurements involving visual judgements by observers of measuring instruments. What is called visual judgement can be divided into (i) judgement by the presence or absence of a stimulus, (ii) judgement by the intensity of the stimulus and (iii) judgement by comparison of the stimulus; we have conducted experiments on measurements involving these three. The results show that in case (i) where judgement relies on the coincidence between the corner of the image and the reference line, individual differences in visual ability are reflected in personal error, and the simpler the visual ability required for judgement, the smaller the degree of personal error in measurement; in the case of (ii)where judgement relies on the observation of outline of the pattern, i.e. judgement through the measurement of the dimensions of an indistinct image, the standards of the judgement of the observer is reflected in personal error; and in the case of (iii), where judgement relies on the visual interpolation of distance between two scale lines, the optical illusion of distance between the lines causes personal error.