A measure of how spread out individual values are around the mean.
SD quantifies the average distance of data points from the sample mean, expressed in the same units as the original measurement. A small SD indicates values cluster tightly around the mean, while a large SD reflects greater variability or heterogeneity in the sample. It underlies many statistical calculations (e.g. confidence intervals, effect sizes) but assumes an approximately normal distribution; for skewed data, median and interquartile range are more appropriate.
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