Variability is one of the more challenging concepts in your course. Let's get a feel for it before we jump into the math.
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The range fails miserably as a proper measure of variability. These videos will show you why that is, and how a new (more complicated) approach give us much better method.
Computational formulas, definitional formulas, population formulas, sample formulas... they're all here!
The standard deviation is not a very intuitive measure. Here are a few videos to help you better grasp the concept.