Thursday, May 28, 2009

BOOK: Software Estimation: Demystifying the Black Art

[This was originally posted at]

I personally have never met a developer who liked giving estimates. Indeed, it's an uphill battle. It's not just that developers don't like working on things which don't compile. It's also that amidst endless ambiguity, changing scope, and technical challenges, some boss pressures you into a specific date when a non-specific feature will be fully complete (with no bugs, of course). However, every aspiring tech lead must eventually fight this battle. Because software estimation is perceived as a "soft science", it's easy for devs to view it as some necessary tax to tolerate in order to get to the "real" work of coding. Therefore, many devs never try to improve their estimating skills, and hence there are a lot of bad techniques out there.

That's why I'm glad that superstar Steve McConnell dumped his wisdom into Software Estimation: Demystifying the Black Art. The book is filled with gems. Here are some of the key tips that I found practical:

  • "Distinguish between estimates, targets, and commitments".
  • Once you've given an estimate and schedule, control the project so that you meet the estimate (reduce scope, juggle staff, prioritize features, etc...). The schedule is not like a basketball that you toss in the air and hope that it makes the shot - rather it's like a football that you personally carry to the end zone.
  • "We are conditioned to believe that estimates expressed as narrow ranges are more accurate than estimates expressed as wider ranges." (pg. 18)
  • Steve emphasizes the "Cone of Uncertainty" - namely that initially, the project has many unknowns, and hence the estimate has a much larger range. However, as the project progresses and more issues become known, the range shrinks. Ironically, it is at the beginner of the project (where everything is most unknown), that many bosses want a stable, singular, estimate. Consider using phase-specific estimation techniques for different stages of the project - some techniques work better at the begininng with higher uncertainty, others work better near the end. Also, within the same project phase, consider using multiple estimation techniques to detect for converging (and hence more reliable) estimates.
  • Always have the people who do the implementation work also provide the estimates.
  • Sometimes, you can use group estimates. However, each developer should come up with their estimates separately, else the dominant person in the group will likely overly-influence everyone else (especially because most devs tend to be introverts). If their estimates differ greatly, then discuss why they're different, and iteratively keep re-estimating until they converge.
  • Collect historical data for past projects, which you can then use to assist with future estimates.
  • Count, Compute, Judge. If you can simply count the size somehow (lines of code, modules, etc...) - then that's best. If you cannot directly count, consider computing from things you can count. The point is you always want to avoid subjective judgments when there's a more objective alternative.
  • Try to estimate based off of objective quantities like historical data and lines of code, as opposed to subjective measurements like developer quality. The boss will heckle the subjective measurements: "Your estimate assumes only average developers, but we're paying for senior devs, so re-calculate with senior devs. Great, that saved us 1 month."
  • When the boss doesn't like the estimate, change inputs, NOT outputs. For example, don't just shave a month off your 5-month estimate because your boss will now be more receptive, rather change the inputs (such as reduce features) so that recalculating the estimate now results in 4 months.
  • Establish a standard estimation procedure for your department. By following an objective set of steps, you (the estimator) have a level of "protection" from all the business sponsors who want a lower estimate. For example, you might say "Our standard procedure, which historically returns estimates within a 20% accuracy, always adds 15% risk buffer for this type of application." Then you aren't the "bad guy" for adding the "extra" 15% (funny how anything that increases an estimate is seen as "extra", not "striving for accuracy").
  • When presenting your estimates (and schedules), don't even bother presenting nearly impossible scenarios. The boss will just assume that you'll be lucky and of course hit the most optimistic number that is mentioned.
  • Always give an honest estimate. Don't lowball the estimate just so that you're boss accepts it - doing so will screw you with an impossible schedule, which will also destroy your credibility ("you can't even hit your own estimate!") If an honest estimate is too high, it is the business sponsor's decision to reject the project, not the developers.

Lastly, consider checking out Construx Estimate.

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