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Name: Cyprian
Status: student	
Age:  N/A
Location: N/A
Country: N/A
Date: 9/17/2005


Question:
Why is standard deviation (error) rather than variance often a more useful measure of variability?



Replies:
While the variance (which is the square of the standard deviation) is mathematically the "more natural" measure of deviation, many people have a better "gut" feel for the standard deviation because it has the same dimensional units as the measurements being analyzed. A similar difference occurs with another statistical variable, the correlation coefficient, usually denoted "R", which measures the correlation between two variables. The more correct measure of such correlation (not cause) is R^2. which is always less than R because by the nature of the formula -1 +/- 1), but that doesn't mean that "A" caused "B". It means that "A" and "B" go up and down together. The "cause" may be some entirely different variable(s) that make "A" and "B" behave that way.

Vince Calder


Mr. G.,

Standard deviation is in the units of the data. Consider a distribution of times, all measured in "seconds". The standard deviation also has "seconds" as its unit. The variance is the square of the standard deviation. The unit for variance would be "seconds-squared". A quantity in the units of the data is much easier to actually relate to the data.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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