what is a z score 
a z score numerically describes a value s relationship to the mean of a group of values and is measured in terms of the number of standard deviations from the mean if a z score is 0 it indicates that the data point s score is identical to the mean score whereas z scores of 1 0 and 1 0 would indicate values one standard deviation above or below the mean in most large data sets 99 of values have a z score between 3 and 3 meaning they lie within three standard deviations above and below the mean 