4.10 Other Sources of Error

The sources of error described in the first nine sections of this chapter are the most common and the most significant, but they are not exhaustive. Here are some of the other ways that error can creep into an estimate:

Unfamiliar business area
Unfamiliar technology area
Incorrect conversion from estimated time to project time (for example, assuming the project team will focus on the project eight hours per day, five days per week)
Misunderstanding of statistical concepts (especially adding together a set of "best case" estimates or a set of "worst case" estimates)
Budgeting processes that undermine effective estimation (especially those that require final budget approval in the wide part of the Cone of Uncertainty)
Having an accurate size estimate, but introducing errors when converting the size estimate to an effort estimate
Having accurate size and effort estimates, but introducing errors when converting those to a schedule estimate
Overstated savings from new development tools or methods
Simplification of the estimate as it's reported up layers of management, fed into the budgeting process, and so on

These topics are all discussed in more detail in later chapters.