Skew (2 of 3)

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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 motor task 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 frequency polygon of test grades on a statistics test where most students did very well but a few did poorly. It has a large negative skew.

negative skew of test scores next previous