The first reaction of many people to uncertain math is to use the same techniques as for certain math. They would either imagine each unknown as an exact mean, or take ‘worst case’ and ‘best case’ scenarios and multiply each one. These two approaches are quite incorrect and produce oversimplified outputs.
This is why I’ve made Guesstimate, a spreadsheet that’s as easy to use as existing spreadsheets, but works for uncertain values. For any cell you can enter confidence intervals (lower and upper bounds) that can represent full probability distributions. 5000 Monte Carlo simulations are performed to find the output interval for each equation, all in the browser.
