Next: The Role of Focus Up: Some Issues of Human Previous: Absolute Judgement of Multidimensional

Measurement versus Detection (Source: Cleveland)

Differentiate distance measurement (absolute) from detection (relative)

Distinguish between 10 graphical perception tasks:

Weber's Law: likelihood of detection is proportional to the relative change, not the absolute change, of a graphical attribute

Stevens' Law: perceived scale in absolute measurements is the actual scale raised to a power. For linear features power is between .9 and 1.1, for area features it is between .6 and .9, and for volume features it is between .5 and .8.

Experiments showed errors in perception ordered as follows (increasing error):