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The spread around a central value should move in the same language as that center. Quartile deviation describes how data sit around the median, using the middle 50% and the median as the center. The natural counterpart for the mean is the standard deviation, which measures typical distance from the mean. It comes from averaging the squared deviations from the mean and then taking the square root, returning to the original units of the data. This pairing is grounded in the fact that the mean minimizes the sum of squared deviations, making standard deviation the most direct measure of dispersion about the mean. The range and variance are related ideas, but the standard deviation is the standard, directly interpretable measure of spread around the mean, not the extremes or a squared-variance form.