Estimating - Make Believe or Science?

Insights | 21 March 2016

Estimating – some see it as an art, others as a science. Whatever type of project we are involved in, and regardless as to whether agile or more traditional approaches are being followed, the need to estimate comes into the mix. How long? How much? How many?

Estimating cannot guarantee accuracy. Consider the definition of the word ‘estimate’ from the Project Management Institute (PMI)®:

“A quantitative assessment of the likely amount or outcome…It should always include some indicator of accuracy (+/- x percent)”
A guide to the Project Management Body of Knowledge – Fifth Edition, PMI®

Estimates are part of our daily project and programme lives, providing a view about overall cost and time so that decisions can be made accordingly.

Approaches to Estimating

There are many different approaches or techniques that can be applied when facing the task of estimating, from the less scientific (‘finger in air’ approach) to those requiring some mathematical calculation. Whatever approach you use, remember a reality check from others never goes astray.

Key Tips in Estimating

  • Involve the experts. Yes, those doing the work should be involved in creating effort estimates. Not only does it provide a ‘hands on’ check but experience shows that such involvement generates greater buy-in from the ‘do-ers’.
  • Optimism Bias Adjustments. In general, people are optimistic (consider that ubiquitous phase ‘she’ll be right’) and often underestimate how long an activity will take.
  • Think Best Case/Worst Case. This gives people a chance to consider the ‘what if’, without immediately jumping to one number as the answer.
  • Comparative/reference class forecasting. Where appropriate past experience and lessons learned can be useful in estimating for the future. Recognise that what you are about to do needs to have some connection with past projects/activities. Some adjustment may be required to reflect the new reality.
  • Watch out for task switching. Remember people working on multiple projects or activities can take longer to complete work due to the downtime involved in switching from one to the other. This is just as relevant for anyone on a project who is a BAU resource as well.
  • Assumptions, Exclusions and Constraints. Ensure that everyone is on the same page, with assumptions clearly outlined, the ‘what you are doing’ (and not doing) articulated, along with any major limitations. Transparency is the key.
  • Provision for other events. Think of meetings and unexpected activities which can undermine original estimates. Yes, meetings take time, can be productive in their own right but should be allowed for.
  • Question estimates. There may be an underlying assumption that you aren’t aware of, the estimate may be based on purely optimistic logic or simply someone’s interpretation of the work. Challenge estimates.

“Estimates should be challenged, as the same work under the same conditions can be estimated differently by various estimators or by the same estimator at different times.”


PMI is a registered marks of the Project Management Institute, Inc.
PRINCE2 is a registered trade mark of AXELOS Limited.

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