Subitizing

Subitizing, coined in 1949 by E.L. Kaufman et al. refers to the rapid, accurate, and confident judgments of number performed for small numbers of items. The term is derived from the Latin adjective subitus (meaning "sudden") and captures a feeling of immediately knowing how many items lie within the visual scene, when the number of items present falls within the subitizing range. Number judgments for larger set-sizes were referred to either as counting or estimating, depending on the number of elements present within the display, and the time given to observers in which to respond (i.e., estimation occurs if insufficient time is available for observers to accurately count all the items present).

The accuracy, speed, and confidence with which observers make judgments of the number of items are critically dependent on the number of elements to be enumerated. Judgments made for displays composed of around one to four items are rapid, accurate and confident. However, as the number of items to be enumerated increases beyond this amount, judgments are made with decreasing accuracy and confidence. In addition, response times rise in a dramatic fashion, with an extra 250–350 ms added for each additional item within the display beyond about four.

While the increase in response time for each additional element within a display is relatively large outside the subitizing range (i.e., 250–350 ms per item), there is still a significant, albeit smaller, increase within the subitizing range, for each additional element within the display (i.e., 40–100 ms per item). A similar pattern of reaction times is found in young children, although with steeper slopes for both the subitizing range and the enumeration range. This suggests there is no span of apprehension as such, if this is defined as the number of items which can be immediately apprehended by cognitive processes, since there is an extra cost associated with each additional item enumerated. However, the relative difference in costs associated with enumerating items within the subitizing range are small, whether measured in terms of accuracy, confidence, or speed of response. Furthermore, the values of all measures appear to differ markedly inside and outside the subitizing range. So, while there may be no span of apprehension, there appear to be real differences in the ways in which a small number of elements is processed by the visual system (i.e., approximately less than four items), compared with larger numbers of elements (i.e., approximately more than four items). Recent findings demonstrated that subitizing and counting are not restricted to visual perception, but also extend to tactile perception (when observers had to name the number of stimulated fingertips).

Read more about Subitizing:  Enumerating Afterimages, Brain Structures Involved in Subitizing and Counting, Educational Applications