graphs (i.e. less that 50 per cent are able to discern differences of this
Measurement
magnitude). In 9 out of 12 trials, the mean response is significantly different
distortion of
from 2 (no difference) at the 0.01 level. These trials relate to the higher levels of graphs in reports
distortion ± 50 per cent, 40 per cent, 30 per cent, 20 per cent and one of the 10
per cent trials. At the 20 per cent level of ``distortion'', between 50 per cent and
75 per cent of subjects perceive a difference and at levels of 30 per cent and
559
above, the proportion rises to 85 per cent and higher.