← Estimating fundamentals

Estimating fundamentals

Estimate accuracy vs precision.

A common misunderstanding in construction estimating: a figure stated to the nearest penny can still be wildly inaccurate. Understanding the difference between accuracy and precision protects both the estimator and the client.

Who this guide is for

Estimators who present cost figures, quantity surveyors reviewing estimates, and anyone who communicates cost information to clients or stakeholders.

Accuracy defined

Accuracy is how close the estimate is to the actual cost at completion. A budget estimate accurate to within ±20% means the real cost could land between £80,000 and £120,000 on a £100,000 estimate. Accuracy is determined by the method used, the information available, and the estimator's experience.

Precision defined

Precision is the level of detail in the stated figure. An estimate of £123,847.63 is highly precise. But if the method was a square metre benchmark, the true cost could still be £90,000 or £150,000. Precision without accuracy is misleading.

Miniature example

Two estimates for the same project: £219,800 (detailed bottom-up, ±8%) and £220,000 (square metre benchmark, ±30%). The second appears less precise but both have overlapping accuracy ranges. Stating £220,000 (±30%) is more honest than £219,800 with no accuracy statement.

Common mistakes

• Stating estimates to the nearest pound or penny when the method cannot support that precision
• Not communicating the accuracy range alongside the estimate
• Choosing a precise-looking method that overshadows a more appropriate but less precise approach