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Question: Distinguish between precision and accuracy.


  • Accuracy means how well your experimental data of measurement agree with the known value of a quantity.
  • Precision means how well your experimental values agree with one another.

Take an example. Let we have a sample of mass and its exact value is 100 kg. We make measurement about the mass of the body three times and get results of 96.6 kg, 96.8 kg and 96.5 kg. The values so obtained are close to one another and we say the scale or measurement is very precise. However, it is not close to the actual value of the body (100 Kg) and hence not accurate.

Then we make the same measurement in some other circumstances, for example with some other scale. We get the results as 99.1 kg, 100.5 kg and 99.7 kg. In this case, the difference in the measured values is relatively large, but close to the actual value of the sample as compared to the previous measurements. We say the instrument is more accurate than the previous one.

Precision in the measurement depends upon the least count (the smallest possible measurement one can make with an instrument or the smallest division of measurement on the instrument) of the instrument. Smaller the least count, more precise the measurement is.

On the other hand, accuracy depends upon the relative error or fractional error (error divided by the measured value of the quantity). If the relative error is smaller the measurement is more accurate.

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