This is a normal distribution, or a bell curve. On the X axis we have the
scores. On the Y axis we have frequencies. Frequencies are how often a score
occurs. It might help to visualize with something like an IQ. Here The mean IQ
is 100. Now we're going to look at deviations. So here we have one standard
deviation. For our IQ, one standard deviation above the mean would be 115. 85,
however, would be one standard deviation below the mean. We see that 68% of the
population will fall between one standard deviation above the mean, one
standard deviation below the mean. Next we see that 95% of the population will
fall with two standard deviations above the mean or two standard deviations
below the mean. Next, we see that 99.7% of the population fall between three
standard deviations above the mean and three standard deviations below the
mean. We have the 68, 95 and 99.7 rule. That means that 68% of the population
is going to fall between one standard eviction below and one standard deviation
above the mean. 95% of the population will fall from two standard deviations
below two standard deviations above the mean. And 99.7% are going to fall from
three standard deviations. Below the mean to three standard deviations above
the mean.
