Shahzad Bhatti Welcome to my ramblings and rants!

February 23, 2009

Software Estimation

Filed under: Project Management,Technology — admin @ 6:04 pm

Software estimation is a difficult art that I am still learning despite developing software for more than twenty years. I have worked on a number of projects that started with some broad vision and manager asked me how many man-months will it take. You feel like a guy who is asked how long will it take you to survey a cave without going inside (see Software Estimates and the Parable of the Cave). So based on some initial requirements, you make up some numbers. But, often that number translates into commitment and some target date. This issue has been also brought up by Software Estimation by Steve McConnell, Manage It by Johanna Rothman, Lean Software Development by Mary Poppendieck and a number of other people. So it must be made clear that your estimate is not the target date.

As a project is always constrained by iron triangle of schedule/cost/functionality or sometime referred to as cost/quality/schedule or cost/resourcs/schedule. It is crucial to find what’s driving the project as also suggested by Johanna Rothman in her book Manage It. I have seen a number of cases where dates were arbitrary picked, sometime referred to as “happy date”. Though, at other times, dates may depend on marketing campaign, seasons, tax time, Olympics, etc. So, you can negotiate between functionality and schedule based on what’s driving the project. Following are some of techniques that I have found useful with estimation:

  • Get the vision and requirements straight – It’s important about the charter, constraints and requirements for the project as any misdirection here would lead to disaster. Luke Hohmann in his book Beyond Software Architecture recommends starting with good vision and mission statement. Johanna Rothman also recommends creating a project charter before starting the project.
  • Probablistic based estimation – Despite the fact, you are often pressured to produce more precise estimates even though they would be inaccurate, it is better to give estimate with some probablity. Both Johanna Rothman and Steve McConnell cite cone of uncertainty, where your estimate becomes more accurate as project progresses.

  • Based on best/worst/most-likely case – use following formula from Steve McConnell’s book can be used when estimates are more accurate:
expected_case = (best_case + (4 * most_likely) + worse_case) / 6

If estimates are not accurate, then Steve McConnell recommends

expected_case = (best_case + (3 * most_likely) + (2 * worse_case)) / 6

Bob Martin also similar formula from his article PERT, CPM, and Agile Project Management:

Mean     = (best_case + worst_case + (4 * most_likely) ) / 6

Variance = ((worst_case_best_case) / 6) ^ 2
  • Iterative development – No matter if you are working on small or large project, the only way to bring some reality and feedback on initial estimate is to develop iteratively starting with highest valued features.
  • T-shirt based estimation – I find t-shirt based estimation useful when estimating with minimal information available. For example, you may have to estimate projects that you can deliver in Q1, Q2, etc and you can order them in small, medium, large and compare them against their business value.
  • Spiking can also help in areas that are new to the team and spending a little time creating walking skeleton or tracer bullet can give you some idea on the size of the effort for the project.
  • Delphi estimation – where PM and team prepares task list, assumptions and estimate in private and reviews them together.
  • Divide and conquer/Decomposition/WBS – as with any large effort, breaking a project into smaller subsystems, components, services and tasks will help estimate better. In general any errors in estimation of smaller tasks will cancel each other.
  • Estimate fine grained tasks – I can rarely estimate with some accuracy for tasks that are longer than a few days so it’s important to estimate only fine grained tasks. XP has a concept of inch pebble and story points that can help in this case. The idea is that each task is either done or not done.
  • Planning poker a technique from Agile Estimating and Planning by Mike Cohn, where each member of the team pick an estimate for a story based on fibonacci numbers, but don’t show until everyone selects some number. The members then pick some average or may ask member with highest or lowest estimates to explain.
  • Historical data – though I rarely see PM track estimates but tracking them can help future projects and new projects can use LOC, man-months, function-points, # of services, files, interfaces, bugs from prior projects for estimation.
  • Schedule chicken – Kent Beck often talks about schedule chicken where you have some some meeting about who is ontrack and you hope there is someone who is behind so that you don’t have to admit you are behind as well. Integrity is big part of the XP and agile methodologies so it encourages transparency and honesty instead of schedule chicken.
  • Better to overestimate than underestimate – programmers often underestimate and though there is risk of student syndrome or Parkinson’s law but it’s better to overestimate.
  • Don’t question developer’s estimate – even though developers tend to underestimate, some managers still question them, which is not a good idea.
  • In XP or Scrum, you use story points, which can be ideal hours or based on some multiplier. These numbers are generally follow fibonacci sequence such as 1, 2, 3, 5, 8, 13, 21.
  • Function points use number of external input/output/queries, internal logical files/external interface files and it can be used as unit of measurements similar to story points.
  • Estimation quality factory (EQF) as proposed by Tom Demarco in his paper A Defined Process For Project Postmortem Review can be used to check how accurate estimates are.
  • Include vacation, sick, holidays as well as non-development activities such as testing, deployment, configuration, migration, etc in your project plan.
  • Scheduling is all about ordering with highest value features. I find rolling-wave scheduling based on milestones useful when planning iterations.


I often find projects turn into death march projects due to overly optimistic estimates and “queen of denial” manager who holds developers’ estimates as commitment and refuses to accept the reality. One way to overcome bad estimation is to adopt iterative development that delivers small features based on the value proposition, which creates biggest value for the business. Another way is to use advice from the Rational Unified Process that uses risk management to prioritize the highest risk tasks first. Though, some managers are keen to accept more work than the team can handle in order to aim high but it takes a courage to say NO. In the end, under-promise and over deliver as it can save your credibility and not to mention unnecessary overtime and stress on the team.

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