A negative Z score indicates that a value is below the mean for the group of values. Z scores show how far away a particular score is from the group mean using standard deviations.
For example, a Z score of -1 shows that the value is one standard deviation below the group mean value. If data are normally distributed, 95 percent of Z scores will fall between -2 and 2 and are considered typical values that represent the sample well. Z scores help statisticians more quickly assess whether a score is typical and aid in the comparison of individual scores from different groups of data. Z scores are also called standardized scores.