Estimates and Uncertainty
In some situations, the factor being estimated may have a broad range of possible outcomes, and other situations may have a much narrower range. Eleven months into a 12-month software project you should have a much better idea of the finish date than you did at project start. The size of that range is sometimes called variance or uncertainty (see Appendix E). The more uncertainty in the factor, the less likely you are to hit the actual outcome with your estimate. Figure 21.2 shows examples of probability distributions with different uncertainties compared to Figure 21.1.
Figure 21.2. Two probability distributions, one with high uncertainty and another with low uncertainty
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