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Question 7: Differentiate between precision and accuracy in measurement?



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.
Precision depends upon the instrument and the techniques used to make the measurement.
Precision is found by the position of the last significant digit in the measurement. For example, the length of a distance is 2930 m. The last significant digit is 3 and the position of 3 in the number is tenth. Therefore, the precision of the measurement is 10 m. Similarly, if we want to find the precision of 0.0210 s, we see the significant digits in the number. There are three sig figs, namely, 2, 1 and 0 (the last one). Now the last sig fig is zero whose position or place in the number is ten thousandth. Therefore, the precision of the measurement is 0.0001.
Since the least count of an instrument gives the last sure or significant digit, therefore, precision depends upon the least count of the instrument of measurement.


Accuracy means how well your experimental data of measurement agree with the known value of a quantity.

Take the above example once again but this time 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 (though not much precise!).
Accuracy of a measurement depends upon the number of significant digits in the answer. If the number of sig figs is greater, the measurement is more accurate. For example, there are two sig figs in 0.025 and hence the accuracy is said to be 2.


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