What is the difference between Precision and Accuracy measurements?

Precision and Accuracy in definition are essential considerations when taking measurements, but they are two different things. While precision measures how close an item is to an actual value (or a known value), accuracy measures its accuracy or reproducibility.

Accuracy refers to how close a value is from its true value. One example of accuracy is the distance an arrow gets from the bullseye center.

Precision refers to how repeatable a measurement can be. A good example of precision is the distance between the second and first arrows, regardless of whether they are near the mark.

To determine if a measurement is accurate and precise enough, the percent error is used.

When you hit a bullseye, accuracy and precision can be compared to accuracy. A bullseye is a target that you hit precisely. This means you are within the target’s center, even though all marks are at different points. A target that is accurately hit means that all hits are precisely spaced even though they are far away from the center. Accurate and repeatable measurements are very close to true values.   

What is the precision of accuracy and precision definitions?
What is the precision of accuracy and precision definitions?

Two common definitions of Accuracy

Accuracy is a measure of how close an item measures to its true value in science, engineering, and math.

The ISO (International Organization for Standardization) uses a stricter definition. Accuracy refers to measurements that produce consistent and true results. An accurate measurement must have no systematic errors or random errors according to the ISO definition. The ISO recommends that precise be used when a measurement can be both exact and precise.

Definition of Precision

Precision shows how consistent measurements are when repeated. Random error is a form observational error that causes precision values to differ from one another.

A basketball player can be thought of as having precision and accuracy definitions. A high level of accuracy is demonstrated by a player who makes a basket every time, even though he hits different parts of the rim. He is precise if he makes fewer baskets but hits the same rim area every time. Players who make free throws that always go in the same direction achieve high levels of precision and accuracy.

Experimental measurements are another example of precision or accuracy. Averaging can show you how close a collection of measurements is to the true value. You can measure the mass of a 51.0-gram sample to get values of 48.6, 48.6, and 48.5 grams. However, this scale is not very precise. Your average measurement is 48.6, which is lower that the true value. Your measurements were still consistent. Your scale should give you values of 50.9, 50.8, 51.5 and 52.0. However, it may not be as accurate as the first balance, but it will still be consistent. Although the average measurement is 51.2, there is a wider range of measurements. If you adjust for the error, a more precise scale is better to use in the laboratory. It is better to calibrate an exact instrument than to use an inaccurate, but accurate, one.

Accuracy, Precision, and Calibration

Is it better to have an accurate measurement recorder or one that is precise? The scale will be accurate if you have weighed yourself three times, and each time, the number is different but close to your true weight. However, it may be more accurate to use a scale that is precise even though it isn’t exact. This would mean that all measurements would be very close to one another, and the true value would be “off” by approximately the same amount. This is a problem with scales that often have a “tare button” to zero them.

Many instruments need calibration, even though scales and balances may allow you to adjust the accuracy or precision of measurements. A thermometer is an example. Thermometers are more reliable within a particular range, but can give inaccurate, but not necessarily imprecise, values beyond that range. Record the deviations from true or known values when calibrating an instrument. To ensure accurate readings, keep a log of calibrations. To ensure precise and accurate readings, many pieces of equipment need to be calibrated periodically.

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