Standard deviation measures how far data values typically fall from the mean. When standard deviation increases, the data are more spread out. This means individual observations tend to be farther from the mean, producing greater variability. A smaller standard deviation means the data values are more tightly clustered around the mean. Option B states the opposite of the correct interpretation. Option C is incorrect because an increase in standard deviation does not necessarily mean the mean increases; center and spread are separate features of a distribution. Option D is also incorrect because variance is the square of standard deviation, so if standard deviation increases, variance increases as well, not decreases. Standard deviation is useful because it is expressed in the same units as the original data, making spread easier to interpret. Study Guide references/topics: standard deviation, variance, spread, measures of variability.

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