Z-score

How it is used

Used to compare scores from different scales, to spot unusual values (|z| > 3 is often flagged) and to look up probabilities in the standard normal table. Converting values to z-scores is called standardising, and a z-score is also known as a standard score.

Example sentences

1. A z-score of 2 means the student scored two standard deviations above the class mean.

2. We converted all test results to z-scores so the two exams could be compared.

Common collocations

• calculate a z-score
• convert to z-scores
• standard score
• z-score table

Related terms

• standard deviationA measure of how spread out values are around the mean; a small value means the data sit close to the mean, a large one means they are widely scattered.
• meanThe sum of all the values divided by the number of values; the usual 'average' that describes the centre of a set of numbers.
• normal distributionA symmetric, bell-shaped distribution in which most values cluster around the mean and become rarer further away from it; its mean, median and mode are equal.
• percentileA value below which a given percentage of the observations fall; for example, 90% of the data lie below the 90th percentile.
• outlierAn observation that lies unusually far from most of the other values in a data set; it may come from an error or from a genuinely unusual case.

More terms: Probability Distributions

• distribution
• skewed
• binomial distribution
• Poisson distribution
• central limit theorem