The mean of a distribution of scores is the average.
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z-scores are distributed according to the standard normal distribution. That is, with the parameters: mean 0 and variance 1.
Variance
99.7% of scores fall within -3 and plus 3 standard deviations around the mean in a normal distribution.
Scores on the SAT form a normal distribution with a mean of µ = 500 with σ = 100. What is the probability that a randomly selected college applicant will have a score greater than 640?
They are said to be Normally distributed.