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Variance. Variance (Ï 2) is a measure of the dispersion of a set of data points around their mean value. In other words, variance is a mathematical expectation of the average squared deviations from the mean. It is computed by finding the probability-weighted average of squared deviations from the expected value. Variance measures the variability from an average (volatility).
Calculating Variance of a Sample. 1  1. Write down your sample data set. 2  2. Write down the sample variance formula. 3  3. Calculate the mean of the sample.  4. Subtract the mean from each data 1  point. 5. Square each result.  6. Find the sum of the squared 1  values. 7. Divide by n - 1, where n is the number of data points.