Difference between Accuracy and Precision

Is accuracy different from precision ??

Yes !!

Below figure on the facing page can help you visualize the difference between precision and accuracy.

Several darts thrown separately at a dartboard may land in various positions, relative to the bull’s-eye and to one another. The closer the darts land to the bull’s - eye, the more accurately they were thrown. The?closer they land to one another, the more precisely they were thrown.

?? In figure A is both accurate and precise: the darts are close to the bull’s - eye and close to each other.

?? In figure B, the set of results is inaccurate but precise: the darts are far from the bull’s - eye but close to each other.

?? In figure C, the set of results is both inaccurate and imprecise: the darts are far from the bull’s - eye and far from each other. Notice also that the darts are not evenly distributed around the bull’s - eye, so the set, even considered on average, is inaccurate.

?? In figure D, the set on average is accurate compared with the third case, but it is imprecise. That is because the darts are distributed evenly around the bull’s - eye but are far from each other.

The terms accuracy and precision mean the same thing to most people. However, in instrumentation their meanings are quite distinct.

Accuracy is the closeness of measurements to the correct or accepted value of the quantity measured

Precision is the closeness of a set of measurements of the same quantity made in the same way.

Same story is applicable in field instrumentation for process parameters like flow, pressure, level, temperature etc.

The three important factors are as below:

1. Accuracy - How close is the measured value to the true value. For e.g., A flow meter being able to measure accurately against a reference standard.

2. Precision - Means repeatability ensuring multiple readings are similar to each other & fitting within a standard deviation.

3. Resolution - The smallest change an instrument can detect.

When choosing instrumentation, high accuracy is of no use, if the instrument is not precise every time a reading is taken.

If you have ever measured something several times, you know that the results can vary. In instrumentation, for a reported measurement to be useful, there must be some indication of its reliability or uncertainty.

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