Z-score : Definition, Formula and Calculation

z score is a measurement of how many standard deviations above the mean or below the mean.

for example in the below image the value 2.9 is 2 standard deviations away from the mean of 2.5, so 2.9 has a z-score of 2.

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3.1 has a z-score of 3.

Z-score formula

z = (x – μ) / σ

Z = standard score
x = observed value
μ = mean of the sample
σ = standard deviation of the sample