Estimating approaches

Top down vs. Bottom up

Estimate whole project duration, cost etc, then break down into stages, tasks etc

Estimate each task, accumulate into stages and whole project


Expert judgement



Question: Who should prepare project estimates?

Expert judgement

Experience important

Compensate for optimism, especially if the estimator is also the manager of the project!

How can we gain experience to become expert?

Wideband Delphi approach

"Plans fail for lack of counsel, but with many advisers they succeed" Proverbs 15:22

Estimating team submit their initial estimates to a coordinator
Coordinator summarises the estimates, and circulates them to the team
Estimating team revise their estimates and resubmit
Process repeats until there is some degree of convergence

Theoretical methods

Formulae and algorithms

  • eg. Halstead's Software Science
  • Basic idea

    Lines of code
    Staff months = -------------

    (Productivity is lines of code per month)


    Comparison with past projects

    1. Establish size of project being planned
    2. Find details of past projects of similar size
    3. Use past actuals to give first estimate
    4. Adjust for known differences


    Formula-based (similarity with Halstead's Software Science)

    Three models for use as more is known about the project

    Function Point Analysis


    Uses metrics


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