Calculating the z-score provides additional information regarding how each subject did overall as the z-score takes dispersion into account.
Z-score indicates how much a given value differs from the standard deviation. For example, the mean of a test could be a 73 and if a student scored an 85, that's great.
However, if the data is not spread out, that 85 could be the highest in the class by 10 points. That's much more information than just 15 points above the mean. This way you can tell when someone not only did well but did exceptionally well in comparison to his or her peers.
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