In this article I will show that the conventional method for calculating uncertainty is not always reliable. In fact, it is generally only exact when the measurement can be represented by a simple linear equation and the input uncertainties are all normally distributed. Whenever the measurement is more complex, there will be errors in the way uncertainties are combined. Using the conventional analytic methods, these errors can be difficult to quantify, although there are some methods that can be used. I will also show how simulation can be a much more reliable approach.

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