What is the difference between accuracy and precision?

A measurement can be accurate without being precise, or precise without being accurate. For a manufacturer to be confident about the quality of their products, they need accuracy and precision. Confused? Read on.

There is an old story that goes like this:

A man walks to work every day past a watch shop. This shop advertises a particular clock, said to be the most accurate ever made. The man is in the army, and one of his responsibilities is a ceremonial duty. He has to fire a gun into the air, every day at noon from the highest point in the city. To make sure he fires the gun at the same time every day, he walks past the clock shop and sets his watch according to this highly accurate clock.

One day, after several years, the man decides to go into the watch shop and ask about this super accurate clock. What’s it made of? How does it work? And finally, how can the proprietor be so sure of its accuracy?

The shop keeper explains: “Every day at noon a soldier fires a gun from the top of the hill, and this clock has never deviated, never lost a second. That’s how I know.”

We could say that both the clock and the firing of the gun are precise because they both produce a repeatable and consistent result. The clock always strikes 12 at the same time the man fires his gun. However, they are not accurate. The measurement in this case is biased because the instruments (clock and gun) are not calibrated correctly.

In non-scientific language we often use the terms ‘accuracy’ and ‘precision’ interchangeably, but  they are not the same. To take a reliable measurement both are equally important.

Even a stopped clock is accurate twice a day

The definition of “accuracy” and “precision”

  • Accuracy can be defined as the closeness of a measured value to a ‘true value’.
  • Precision is defined as the ability of a measurement to be consistently reproduced.

Accuracy means getting the correct answer. Precision means getting the same answer every time.

If a student adds up 1 + 1 and reaches an answer of 2 they are accurate. To be accurate and precise the same student would need to add up 1 + 1 several times and always find the same answer. However, if that same student repeatedly finds an answer of 3, they are (of course) not accurate, but they are precise.

Returning to our analogy of clocks. Imagine an unscrupulous manufacturer who claims to sell a perfectly accurate watch but fails to mention that the watch never gives a precise reading. One day it tells the correct time. The next day it’s a few seconds slow. Another day it’s fast. If you take an average reading over a month, the watch is perfectly accurate, but at any one occasion the owner will never know precise time.

Accuracy is not the same as precision

Why do accuracy and precision matter?

We all make measurements every day. Simply looking at a clock to tell the time is a measurement. Every time you step on the bathroom scales you are taking a measurement. Boil the kettle and when it turns off automatically at 100 degrees, that’s a measurement.

But these two concepts are far more significant than the temperature of your morning cup of tea. Manufacturers need to have confidence in their components. Reputations, contracts, businesses, even lives might depend on it. By striving to improve accuracy and precision in manufacturing, engineers can make safer products. They can reduce waste materials. They’ll make better, more reliable products that last longer and cost less to maintain.

If you’re interested in reading more articles on basic and applied metrology concepts there are lots of resources available. Visit our our popular Metrology 101 materials here. There is also the National Physics Laboratory’s beginners guide to measurement, and for a more solution based approach check out our fascinating “Can I Measure It?” video series.

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