Estimation Safety Tips

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the total by six. This results in a weighted average that reflects the estimate range.

Unfortunately, the people to whom you present a range of estimates might select the lowest value of the range as the one they want to hear. A manager who goes with the minimum estimate has chosen to manage his project at a low confidence level, with only a small likelihood of delivering on project commitments. There's no defense against unreasonable people who reject thoughtful estimates, but at least get into the habit of acknowledging the intrinsic uncertainty associated with all prognostications.

Estimation Safety Tip #No. 5: Incorporate contingency buffers into estimates.
Nothing goes exactly as planned. Therefore, build some contingency time into the estimate for each chunk of work you do. These contingencies will help you accommodate any unplanned tasks that might arise, and they'll help compensate for estimates that turn out to be too low. Contingency buffers also help you cope with risks that materialize into problems. If you don't leave any slack in your schedule, the first unexpected situation you encounter will demolish your plans. Work to meet the nominal estimates, but commit externally to the estimates plus the contingencies.

A manager who discards thoughtfully planned contingency buffers is making several assumptions:

  • All estimates are perfect
  • No risks will materialize into problems
  • There will be no growth in the requirements
  • Nothing unexpected will happen

These are terrible assumptions! To help a manager avoid these assumptions, you could point out some unexpected experiences from previous projects. Ask the manager if there is any reason to believe that the new project will be different and that none of those unpleasant experiences will be repeated. If no reason can be provided, then the contingency buffers should stay.

These safety tips might not help you generate estimates with which all project stakeholders will be thrilled, but they will help you and your team deal realistically with what the future might bring.

About the author

Karl E. Wiegers's picture Karl E. Wiegers

Karl Wiegers, Ph.D., is the Principal Consultant at Process Impact in Portland, Oregon, and the author of Software Requirements (Microsoft Press, 1999) and Creating a Software Engineering Culture (Dorset House, 1996). You can reach him at www.processimpact.com. You can find more than 35 of Karl's articles at www.processimpact.com/pubs.shtml

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