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The mean is the average value and the standard deviation is the variation from the mean value.
Residual point
For a population the mean and the expected value are just two names for the same thing. For a sample the mean is the same as the average and no expected value exists.
mean: average
z-score of a value=(that value minus the mean)/(standard deviation). So a z-score of -1.5 means that a value is 1.5 standard deviations below the mean.