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

SECTION 2.3 | Frequency Distribution Graphs 45
5
FIGURE 2.6
An example of a frequency
distribution polygon for
grouped data. The same
set of data is presented in a
frequency distribution table
and in a polygon.
Frequency
4
3
2
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Scores
X
12–13
10–11
8–9
6–7
4–5
f
4
5
3
3
2
Bar Graphs A bar graph is essentially the same as a histogram, except that spaces are
left between adjacent bars. For a nominal scale, the space between bars emphasizes that
the scale consists of separate, distinct categories. For ordinal scales, separate bars are used
because you cannot assume that the categories are all the same size.
To construct a bar graph, list the categories of measurement along the X-axis and then
draw a bar above each category so that the height of the bar corresponds to the frequency
for the category. An example of a bar graph is shown in Figure 2.7.
■ Graphs for Population Distributions
When you can obtain an exact frequency for each score in a population, you can construct
frequency distribution graphs that are exactly the same as the histograms, polygons, and
bar graphs that are typically used for samples. For example, if a population is defined
as a specific group of N = 50 people, we could easily determine how many have IQs of
X = 110. However, if we were interested in the entire population of adults in the United
States, it would be impossible to obtain an exact count of the number of people with an
IQ of 110. Although it is still possible to construct graphs showing frequency distributions
for extremely large populations, the graphs usually involve two special features: relative
frequencies and smooth curves.
20
FIGURE 2.7
A bar graph showing the distribution of
personality types in a sample of college
students. Because personality type is a
discrete variable measured on a nominal
scale, the graph is drawn with space
between the bars.
Frequency
15
10
5
0
A B C
Personality type