A z score is used when the sample size is generally greater than 30, thus assuming normal distribution. Z scores are very useful in determining the relative ability of an individual when comparing two differently formatted tests. For example: A person in high school who scores 13 out of 20 on a AP Physics test, which has a mean of 10 and standard deviation of 3 test would thus have a z score of 1. Another student, who scores 90 out of 100 on a basic arithmetic exam with a mean of 80 and standard deviation of 20, would only be 1/2 standard deviation above the mean. Thus the student with the higher Z score (greater amount of standard deviations above the mean) would have performed relatively better.