The graph to the right is a
visual depicition of the
bell curve where the
mean is the middle of the
curve. Increments to the
right and left of the mean
are standard deviations
and provide information
on how far below or
above a score is from the
mean. The farther a score
is from the mean, the
more atypical it is.
Psychologists and teachers commonly use these scores when describing test
performance results. For example, you may have heard of someone having an IQ score
of 100. This called a standard score because you can see that 100 is at the exact
center of the curve. Z-scores, T scores, and scaled scores express the same thing that
standard scores do, but do so based on a different numerical system with different
means and standard deviation units as shown above. The percentage of people who
score is each standard deviation unit is noted in the bell curve above with % values.
The percentile rank allows you to determine an
individual's position in relation to a sample of
other individuals. More specifically, the percentile
rank is the point in a distribution at or below which
the scores of a given percentage of individuals
fall. For example, a person with an IQ score of
120 (and a percentile rank of 91) has scored as
well or better than 91 percent of people in the
normal sample. The table below is intended for
general usage by health care providers and the
general public. For other measurements, please