Another plan doomed by detail?

The plan is perfect, even down to the detail we could tell you what the team was doing after lunch on the 2nd Thursday of November next year.

But is this ever going to be true? Although many would point to the PMI as the source of the belief in hard estimates this early in a project, is that really fair? Looking at A Guide to the Project Management Body of Knowledge (PMBoK) 4th Edition, the PMI defines estimates in the Glossary as:

Estimate [Output/Input]. A quantitative assessment of the likely amount or outcome. Usually applied to project costs, resources, effort, and durations and is usually preceded by a modifier (i.e., preliminary, conceptual, feasibility, order-of-magnitude, definitive). It should always include some indication of accuracy (e.g., ± x percent).

This phrase, “quantitative assessment,” comes up in several places that mention estimates. For example, in Section 7.1 Estimate Costs the PMBoK states:

Cost estimates should be refined during the course of the project to reflect additional detail as it becomes available. The accuracy of a project estimate will increase as the project progresses through the project lite cycle. Hence cost estimating is an iterative process from phase to phase. For example, a project in the initiation phase could have a rough order of magnitude (ROM) estimate in the range of ±50%. Later in the project, as more information is known, estimates could narrow to a range of ±10%. In some organizations, there are guidelines for when such refinements can be made and the degree of accuracy that is expected.

So if the PMI have ROM estimates of ±50%, and only later will we get down to ±10% why do we assume we’re 100% accurate on day 1?
This narrowing is the Steve McConnell’s “Cone of Confusion”. A version from Mitch Lacey’s 2012 MSDN article on Estimating is shown below:
Cone of Uncertainty

Yet many clients expect this level of detail and certainty upfront, and therein lies the dilemma.

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