In the place of the ranges used for qualitative risk analysis, quantitative assessment uses a numerical percentage, a specific fraction between 0 and 100 percent. There are three ways to estimate probabilities:
A prediction, based on a mathematical model
An empirical calculation using historical data
Selection of a number based on the best analysis available
Some risk situations may be modeled and in other cases there may be sufficient data to use in statistical forecasting. However, because many project risks are unique, complex, and rarely occurring, the third technique (better known as guessing) is most frequently used. Quantitative risk probability estimates are often inexact.