Developmental psychology.pdf

520 Psychology and Society
Figure 19.5
Skewed Distribution. When the
curve is not symmetrical, the mode,
median, and mean may be quite
different from one another. The
shaded area indicates the scores
which create the skew, here called a
positive skew because it involves a
spread of additional high scores.
о
с
- Median
Score
"One type of skewed distribution
occurs in the arrival of students at
a cafeteria. Most people get there
just a few minutes before the
cafeteria opens, and then a few
continue to drift in during most of
the rest of the dinner hour.
At other times, the scores pile up at one end or the other, and the distribution
is clearly nonnormal. This asymmetrical curve is called a skewed curve.* Here the three
measures of central tendency provide different indications of the typical score. Hence
one must be careful in selecting any measure of central tendency as representative of
the entire distribution (Figure 19.5).
Measures of Variability
In addition to the average or typical score, office managers want to know about differences
among workers. Suppose a manager is planning to develop an on-the-job training
program for graduates from a word-processing course. Which school should be
chosen? Schools X and Y both claim a mean rate of 50 words per minute, as shown
in these 10 scores from each institution:
X 51 48 45 49 53 52 50 47 49 56
Y 58 42 52 40 59 46 54 50 41 58
Computation of the means verifies these claims, but closer examination shows an important
difference between the two institutions. The students from X are highly predictable,
ranging from 45 to 56 words per minute, all close to the average. Those from
Y are highly variable. Some students perform quite well, others do poorly, achieving
only 40 and 41 words per minute. In choosing to affiliate with one school or the other,
it is useful to know not only the typical score but also the extent to which students vary
from this typical score. In this case the office manager may choose to affiliate with
school X, for these students are more homogeneous in ability and therefore perhaps
easier to instruct in on-the-job training.
In other words, in addition to some measure of central tendency, the office
manager needs some measure of difference or dispersion. This measure, called a measure
of variability, indicates the degree to which the scores differ from some typical
score. Measures of central tendency and measures of variability are used together in
describing a group of scores.
Range and Standard Deviation The simplest indicator of variability is the range,
which is the difference between the lowest and highest scores. The range for X is 11
and for Y it is 19. Like the mode, the range is unstable, for it takes into account only
a few scores in the group.