Statistics for the Behavioral Sciences by Frederick J. Gravetter, Larry B. Wallnau ISBN 10: 1305504917 ISBN 13: 9781305504912

46 CHAPTER 2 | Frequency Distributions
FIGURE 2.8
A frequency distribution showing the relative
frequency of females and males in the United
States. Note that the exact number of individuals is
not known. The graph simply shows that there are
slightly more females than males.
Relative frequency
Females
Males
Relative Frequencies Although you usually cannot find the absolute frequency for
each score in a population, you very often can obtain relative frequencies. For example,
no one knows the exact number of male and female human beings living in the United
States because the exact numbers keep changing. However, based on past census data and
general trends, we can estimate that the two numbers are very close, with women slightly
outnumbering men. You can represent these relative frequencies in a bar graph by making
the bar above female slightly taller than the bar above male (Figure 2.8). Notice that the
graph does not show the absolute number of people. Instead, it shows the relative number
of females and males.
Smooth Curves When a population consists of numerical scores from an interval or
a ratio scale, it is customary to draw the distribution with a smooth curve instead of the
jagged, step-wise shapes that occur with histograms and polygons. The smooth curve indicates
that you are not connecting a series of dots (real frequencies) but instead are showing
the relative changes that occur from one score to the next. One commonly occurring
population distribution is the normal curve. The word normal refers to a specific shape that
can be precisely defined by an equation. Less precisely, we can describe a normal distribution
as being symmetrical, with the greatest frequency in the middle and relatively smaller
frequencies as you move toward either extreme. A good example of a normal distribution
is the population distribution for IQ scores shown in Figure 2.9. Because normal-shaped
Relative frequency
FIGURE 2.9
The population distribution
of IQ scores; an example of
a normal distribution.
70 85 100 115 130
IQ scores