Figure 2.12 illustrates how a pdf is approximated by deviding its domain into a set of intervals. The X-axis shows the multiples of the interval width
(
,
,
...). These values represent the mean values of the intervals to which the probabilities will be associated. The figure shows how the probability of the third interval is computed. The interval borders are given by
and its mean value is
. The probability
is the striped area under the graph. So, all values lying in this area are associated to the value
.