called themedian. In Figure 2.7 the median family income in 1983 for non-
whites was about $17,500, compared to a median of about $22,000 for white
families. To get the median, we start at the 50% point on the vertical axis and
go horizontally until meeting the cumulative frequency graph; the projection of
this intersection on the horizontal axis is the median. Other percentiles are
obtained similarly.
The cumulative frequency graph also provides an important application in
the formation of health norms (see Figure 2.8) for the monitoring of physical
progress (weight and height) of infants and children. Here, the same percentiles,
say the 90th, of weight or height of groups of di¤erent ages are joined by a
curve.
Example 2.6 Figure 2.9 provides results from a study of Hmong refugees in
the Minneapolis–St. Paul area, where each dot represents the average height of
five refugee girls of the same age. The graph shows that even though the refu-
gee girls are small, mostly in the lowest 25%, they grow at the same rate as
Figure 2.8 (a) Weight and (b) height curves.TABULAR AND GRAPHICAL METHODS 69