The precision of numerosity discrimination in arrays of random dots.

The precision of making discriminations between the numbers of dots in a pair of irregular arrays was measured. The results fit the assumption that the observer adds intrinsic variance to whatever variance is present in the numbers displayed, the errors depending upon the sum of the two. We found no evidence for incomplete use of the sample of information presented, other than this observer variance. Its value increases as about the 0.75 power of the mean number of dots in a display, except for numbers up to about 20 where it changes much more rapidly. Decreased irregularity in the arrangement of the dots decreases observer variance, but it is little affected by large variations in average density of dots per unit area, and is also little changed by making the dots vary irregularly in brightness.