Skew (2 of 3)
Distributions with positive skew are more common than distributions with
negative skews. One example is the distribution of income. Most people make
under $40,000 a year, but some make quite a bit more with a small number
making many millions of dollars per year. The positive tail therefore extends
out quite a long way whereas the negative tail stops at zero.
For a more psychological example, a distribution with a positive skew
typically results if the time it takes to make a response is measured.
The longest response times are usually much longer than typical response
times whereas the shortest response times are seldom much less than the
typical response time. A histogram
of the author's
performance on a perceptual
in which the goal is to move the mouse to and click on
a small target as quickly as possible is shown below. The X axis shows
times in milliseconds.
Negatively skewed distributions do occur, however. Consider the following
of test grades on a statistics
test where most students did very well but a few did poorly. It has a
large negative skew.