Risk in software estimation

Plus-or-minus qualifiers. Use plus-or-minus style estimates to indicate both the amount and the direction of uncertainty in the estimate. Even when you have been forced into promising the software in an unrealistic time frame, you can let those around you know how risky the schedule is by presenting your estimates in plus-or-minus style. An estimate of 6 months, + ½ month, –½ month says that the estimate is quite precise and that there’s a good chance of meeting the estimate. An estimate of 6 months, + 3 months, –2 months says that the estimate isn’t very precise and that there’s less chance of meeting the estimate. An estimate of 6 months, + 6 months, –0 months says that the estimate is quite optimistic—probably unrealistic.

Steve McConnell, Rapid Development: Taming Wild Software Schedules

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