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Knowledge

Accuracy vs Precision in Laboratory Testing

Learn the difference between accuracy and precision and why both are important in laboratory measurement performance.

4 min read Laboratory Quality Assurance #concentration
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Learning Objectives

By the end of this resource you should be able to:

- Define accuracy and precision.
- Distinguish systematic displacement from random variation.
- Explain how a method can be precise but not sufficiently accurate.
- Recognize why method-specific evaluation criteria are needed.

Accuracy vs Precision in Laboratory Testing

Overview

Accuracy and precision are related but different concepts in measurement science. Understanding the distinction is essential when evaluating laboratory examination procedures.

Accuracy

In measurement science, accuracy refers to the closeness of agreement between a measurement result and an accepted reference or true value, with the exact terminology depending on the measurement framework.

Accuracy should not be reduced to a single statistical property.

Precision

Precision describes the closeness of agreement between repeated measurement results obtained under specified conditions.

Precision can be considered under different conditions, such as:

  • Repeatability
  • Intermediate precision
  • Reproducibility

The relevant condition depends on the evaluation being performed.

A simple conceptual example

Imagine repeated measurements of the same material:

  • Results tightly grouped around the accepted value: good precision and good trueness/accuracy.
  • Results tightly grouped but displaced from the accepted value: good precision but poor trueness.
  • Results widely scattered around the accepted value: poorer precision.
  • Results widely scattered and displaced: poor precision and poor trueness.

This illustrates why repeatability alone cannot establish complete measurement performance.

Bias and imprecision

Two useful concepts are:

Bias: systematic difference between the expectation of measurement results and an accepted reference value.

Imprecision: variation among repeated measurement results.

A method evaluation may therefore consider both systematic and random components of measurement error.

Laboratory application

When evaluating a method, the laboratory should define appropriate performance characteristics and acceptance criteria for the intended use.

Possible considerations include:

  • Precision
  • Bias or trueness
  • Analytical measurement range
  • Detection capability where relevant
  • Interference
  • Carryover where relevant
  • Comparability
  • Stability
  • Other method-specific characteristics

Important distinction

Accuracy and precision should not be used as interchangeable words in a technical laboratory report. The laboratory should follow the terminology required by its adopted measurement and quality framework.

Key points

  • Precision concerns agreement among repeated measurements.
  • Accuracy is a broader concept concerning closeness to the accepted or true value.
  • A method can be precise while having systematic bias.
  • Method evaluation should use predefined, fit-for-purpose criteria.

References

  • ISO 5725 series. Accuracy (trueness and precision) of measurement methods and results.
  • ISO 15189:2022. Medical laboratories — Requirements for quality and competence.