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

SECTION 3.5 | Selecting a Measure of Central Tendency 87
5
4
FIGURE 3.8
A frequency distribution with
one extreme score. Notice that
the graph shows two breaks in
the X-axis. Rather than listing
all of the scores for 0 –100,
the graph skips directly to the
lowest score, which is X = 10,
and then breaks again between
X = 15 and X = 100. The
breaks in the X-axis are the
conventional way of notifying
the reader that some values
have been omitted.
Frequency
3
2
1
10 11 12 13 14 15
100
Number of errors
skewed, with a small segment of the population earning incomes that are astronomical.
These extreme values distort the mean, so that it is not very representative of the salaries
that most of us earn. As in the previous example, the median is the preferred measure of
central tendency when extreme scores exist.
Undetermined Values Occasionally, you will encounter a situation in which an individual
has an unknown or undetermined score. This often occurs when you are measuring
the number of errors (or amount of time) required for an individual to complete a task.
For example, suppose that preschool children are asked to assemble a wooden puzzle as
quickly as possible. The experimenter records how long (in minutes) it takes each child
to arrange all the pieces to complete the puzzle. Table 3.5 presents results for a sample of
n = 6 children.
TABLE 3.5
Number of minutes
needed to assemble a
wooden puzzle.
Person
Time (Min.)
1 8
2 11
3 12
4 13
5 17
6 Never finished
Notice that one child never completed the puzzle. After an hour, this child still showed
no sign of solving the puzzle, so the experimenter stopped him or her. This participant has
an undetermined score. (There are two important points to be noted. First, the experimenter
should not throw out this individual’s score. The whole purpose for using a sample is to
gain a picture of the population, and this child tells us that part of the population cannot