Z-score

Definition

Z-score in plain English

A z-score answers the question 'is this number normal or unusual, and by how much?' — but in a fair way that works for any kind of data. It measures how many standard deviations (the typical amount values stray from average) a value sits above or below the mean (the average). For example, if the average test score is 70 with a standard deviation of 10, a score of 90 has a z-score of +2 — two steps above average, genuinely high; a 65 has a z-score of -0.5, just slightly below. Because it strips away the original units, a z-score lets you compare wildly different things — say, how impressive a height is versus how impressive a salary is — on one common scale.

Related MCAT terms

• Critical Analysis and Reasoning Skills (CARS)
• Confidence interval
• Confounding variable
• Correlation coefficient
• Effect size
• Hypothesis
• p-value
• Passage-based reasoning
• Power
• Skills for Critical Analysis and Reasoning (CARS)