Accuracy vs. Precision

The words accuracy and precision are often used almost interchangeably in colloquial usage. However, when it comes to measurement system analysis their meaning, interpretation and usage is widely different. It is important to understand these characteristics since they form a part of a good measurement system.

Defining Accuracy

A measurement system is said to be accurate, if the average of its observed values is close to the actual value. In other words, values must be observed and their mean be calculated. This mean should then be compared to a standard value. The closer the mean is to the standard value, the more accurate the system is.

Accuracy, therefore pertains to the mean of the observed data. A good analogy to explain this concept would be that of darts. If a number of darts are thrown on the dartboard and if all of them center around the bull’s eye, then the throws were accurate!

Defining Precision

A measurement system is said to be precise, if the observed values are in close proximity to each other. In other words, the observed values must lie within a small distance from each other. Precision, therefore is a function of the standard deviation of the data that has been observed. The less the standard deviation, the more precise the measurement system is.

Extending the darts analogy, if many darts were thrown at the dartboard and if they were in close proximity of one another, regardless of how far they were from the bull’s eye, they were precise!

The characteristics of a precise system are repeatability and reproducibility. They have been explained below:

  • Repeatability: Repeatability is the ability of a system to produce measurements that are in close proximity to each other when the same person measures it using the same equipment. The factor being varied here is time. A repeatable system ensures that measurements taken over time are consistent.

  • Reproducibility: Reproducibility is the ability of the system to produce consistent measurements when different operators are using different equipment to measure it. A strong system must be capable of giving consistent measurements regardless of who is operating it and with what.

Four Possible States of a Measurement System

It is important to realise that when since accuracy and precision have different meanings in the context of measurement systems, there are 4 possible states that a measurement system can have in this regard. The measurement system under consideration maybe:

  • Both accurate and precise
  • Accurate but not precise
  • Precise but not accurate
  • Neither accurate nor precise

When accuracy and precision are present in the system together, it gives measurements that are close to the standard value and to each other. This is the desired state of affairs that every measurement system eventually works toward.

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