Complete accuracy is impossible to achieve in estimating. A high degree of accuracy is what a project manager strives for while developing estimates. Reviewing the past performance improves the forecasts of the future. Comparing actual performance to estimates is essential to refining the estimating model. A project manager cannot control all the project variables and cannot predict the future. For example changing technology, weather, regulations may result in change of plan.
One of the important steps a project manager can take is to build estimates with complete knowledge of specifications of implementations. No amount of good business practices can replace the detailed blueprint. The blueprint gives the people developing estimates the right tool to do their job. Best people to make estimates are experienced with estimating and people who will actually work on the project. Though all projects have their own characteristics but there are enough similarities in a typical project life cycle to gain from experienced people. Professional estimators bring the expertise and finesse which is generally beyond the scope of a project manager.
Phased estimating, apportioning, parametric estimating and bottom estimating are the various estimating techniques used by project managers. Phased estimating requires cost and schedule resource estimates for one phase of a project at a time. At the end of the first phase, a new estimate for the second phase is developed. Bottom-up estimating is considered the most accurate estimating as it takes into consideration the detailed tasks and work packages.
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