42 FREQUENCY TABLES AND THEIR GRAPHSFigure 4.1
workers. (b) High-altitude mine workers.
Distributions of hemoglobin levels of mine workers. (a) Low-altitude minemisleading impression. Table 4.6 shows the age distribution of 302 hospital deaths
from scarlet fever in 1905-1914. Both the frequencies and proportions are given.
If a histogram had been plotted directly from the frequencies in the table, it would
leave the impression that deaths from scarlet fever rise sharply from 9 to 10 years
of age. This erroneous impression arises simply from the fact that there is a change
in the length of the class interval. The eye seems naturally to compare the various
areas on the graph rather than just their heights. The number of deaths from 10 up to
15 years is 14. Fourteen deaths is larger than the number of deaths at either age 8 or
- If we plotted a bar 14 units high for the number of deaths from 10 up to 15 years,
we would make it appear that the number of deaths is greater among this age group
than among either 8- and 9-year-olds.
To correct this, we plot the number of deaths per year of the interval rather than
just the number of deaths. Since there are 14 deaths in the interval extending from
10 to 15 years of age, there were 2.8 deaths per year of the interval. A single bar 2.8