Accuracy and Precision - The Art of Measurement

Measurement is essential for us to understand the external world and through millions of years of life, we have developed a sense of measurement. You intuitively know how much time has passed or how much distance you have walked. Primitive societies needed rudimentary measures for many tasks such as the construction of dwellings of an appropriate size and shape. Measurements require tools that provide scientists with a quantity.

The problem here is that the result of every measurement by any measuring instrument contains some uncertainty. This uncertainty is referred to as Error.

What is Accuracy And Precision?

Accuracy:The closeness of a measured value to the actual value of the object being measured is called as the accuracy of a substance. For instance, if in the lab, you obtain a weight measurement of 3.2 kg for a given substance, but the actual or known weight is 10 kg, then your measurement is not accurate.

Precision:The closeness of two or more measurements to each other is known as the precision of a substance. From the above-given example, we can figure out that, if you weigh a given substance five times, and get 3.2 kg each time, then your measurement is very precise. Precision is independent of accuracy. The below example will tell you about how you can be precise but not accurate and vice versa. Precision is sometimes separated into:

  • Repeatability
    The variation arising when the conditions are kept identical and repeated measurements are taken during a short time period.
  • Reproducibility
    The variation arising using the same measurement process among different instruments and operators, and over longer time periods.

Conclusion

Accuracy is the degree of closeness between a measurement and the measurement’s true value. Precision is the degree to which repeated measurements under the same conditions are unchanged.

Since you are here, you might want to check out the following articles:

  • Errors in Measurement
  • Measurement of length

Accuracy and Precision Examples

A good analogy for understanding accuracy and precision is to imagine a football player shooting at the goal. If the player shoots into the goal, he is said to be accurate. A football player who keeps striking the same goalpost is precise but not accurate. Therefore a football player can be accurate without being precise if he hits the ball all over the place but still scores. A precise player will hit the ball to the same spot repeatedly, irrespective of whether he scores or not. A precise and accurate football player will not only aim at a single spot but also score the goal.

 

The top left image shows the target hit at high precision and accuracy. The top right image shows the target hit at a high accuracy but low precision. The bottom left image shows the target hit at a high precision but low accuracy. The bottom right image shows the target hit at low accuracy and low precision. 

More Examples

  • If the weather temperature reads 28 °C outside and it is 28 °C outside, then the measurement is said to be accurate. If the thermometer continuously registers the same temperature for several days then the measurement is also precise.
  • If you take the measurement of the mass of a body of 20 kg and you get 17.4,17,17.3 and 17.1, your weighing scale is precise but not very accurate. If your scale gives you values of 19.8, 20.5, 21.0 and 19.6, it is more accurate than the first balance but not very precise.