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:<math>\sqrt{ \frac{ n\Sigma x^2 - \left( \Sigma x \right) ^2 }{n^2} } \ . </math>
However, the first form keeps better numerical accuracy for large values of ''x'', because squares of differences between ''x'' and ''x''<sub>av</sub> leads to less round-off than the differences between the much larger numbers Σx<sup>2</sup> and (Σx)<sup>2</sup>. The built-in Excel function STDEVP(), however, uses the less accurate formulation because it is faster computationally.<ref name=
{{cite book |title=Advanced Excel for scientific data analysis |author=Robert de Levie |publisher=Oxford University Press |year=2004 |isbn=0195152751 |url=http://books.google.com/books?id=IAnO-2qVazsC&printsec=frontcove|pages=45–46}}</ref>
</ref>
===Round-off error===
User computations must be carefully organized to insure round-off error does not become an issue. An example occurs in solving a [[quadratic equation]]:
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