stats
posted by kelly .
Why must you square the deviation scores when computing a standard deviation using the definitional formula?
Why is the computational formula easier to use when computing variance and standard deviation?

The sum of the deviations (unsquared) from the mean should always = zero, since the mean serves as a fulcrum (balance point) for the distribution.
You need to square the sum of the scores rather than the sum of the squared deviations (squaring each scores deviation from the mean).