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Accuracy and Precision in Chemistry

04th May 2019 @ 5 min read

Basic Chemistry

In measurements and sampling, the two terms accuracy and precision are very important to understand. We often use these words interchangeably in regular conversation, but they bear different meaning here. Accuracy refers to the closeness between the measured value and actual value of a quantity. While precision indicates closeness among the measured values of a quantity. Accuracy indicates how much the measured value agrees with actual value, and precision indicates how close the measured values agree among themselves. The results which are easily reproducible have good precision. Accuracy and precision are best explained by darts throwing game.

Accuracy vs Precision
Accuracy vs Precision (Darts throwing game)

When darts are landed on the dartboard away from the target point, we say it has low accuracy. But when they are closed to the target point, we say it has high accuracy. If darts are closed to each other, then it is high precision. Similarly, when they are spread out, it is low precision. In the above figure, the four possible conditions are illustrated.

  1. When darts are closed to each other as well as to the target point, it is high precision and high accuracy. This is the desired condition. Chemists always try to get results with great precision and accuracy.
  2. In 2nd condition, all the darts are closed to each other but away from the target. Such a condition has low accuracy and high precision. In chemistry labs, such condition arises when there is an error in measuring system itself, for example, weighing machine is not properly calibrated. Defect in a measuring instrument always results in poor accuracy. It is an undesirable condition.
  3. When darts are closed enough to the target but not among themselves, the condition is high accuracy with low precision. This is also an undesirable result. In laboratories, when an experimenter lacks knowledge of the experiment or he/she is not conducting the experiment in a procedural manner, such errors emerge.
  4. In the last condition, there is neither accuracy nor precision. This is because all the darts are spread-out away from each other and from the target point. This is a highly unacceptable condition.

In laboratories, the regular calibration of instrumentations is necessary to maintain the accuracy and precision of measurements.

Consider an experiment involving measuring the mass of copper sulphate whose actual weight is 2.000 g. Student A measures the mass of the salt thrice on weighing machine, he gets values 2.013 g, 2.023 g and 2.002 g. The same measurements are repeated by student B, student C, and student D on different weighing machines. All the readings are reported in the table below.

Accuracy vs Precision (experimental data)
StudentReading 1Reading 2Reading 3Comments
A2.0132.0232.002High accuracy, high precision
B1.9011.9021.910Low accuracy, high precision
C1.9901.9852.015High accuracy, low precision
D1.8001.9212.203Low accuracy, low precision

From the above table, we can conclude that student A has performed the experiment correctly and precisely. For student B, it might be the weighing machine is not properly calibrated. Student C is not following proper steps while taking the reading like not using vented balance safety enclosures. Finally, student D is the worst case of all the above.

Weighing machine
Weighing machine with vented safety balance enclosure
[Image source: Public domain]

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