In common usage variance is the difference between a pre-established measure and an actual measure .(Wideman Comparative Glossary of Common Project Management Terms v2.1 )

Other Definitions

Variance – A discrepancy between the actual and planned performance on a project, either in terms of schedule or cost .(Wideman Comparative Glossary of Common Project Management Terms v2.1 )

Variance – The variance is one of several indices of variability that statisticians use to characterize the dispersion among the measures in a given population. To calculate the variance of a given population, it is necessary to first calculate the mean of the scores, then measure the amount that each score deviates from the mean and then square that deviation (by multiplying it by itself). Numerically, the variance equals the average of the several squared deviations from the mean. The square of the standard deviation.

J. Jerrald Hayes
I am ex-architectural woodworker and general contractor turned IT, Business and Project Management consultant, software developer wannabe senior division triathlete and ski racer, Yankee fan and founder of Paradigm-360.com, 360 Difference Mac4Construction.com,iOS4Construction.com and now TheBuildingAndRemodelingWiki.com too.
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