Accuracy, Precision, and Significant Figures

Read this text to learn more about uncertainty, accuracy, precision and significant figures.

Introduction

An old rusted double-pan balance is shown with a weighing stone on one pan.

Figure 1.22 A double-pan mechanical balance is used to compare different masses. Usually an object with unknown mass is placed in one pan and objects of known mass are placed in the other pan. When the bar that connects the two pans is horizontal, then the masses in both pans are equal. The "known masses" are typically metal cylinders of standard mass such as 1 gram, 10 grams, and 100 grams.

A digital analytical balance.

Figure 1.23 Many mechanical balances, such as double-pan balances, have been replaced by digital scales, which can typically measure the mass of an object more precisely. Whereas a mechanical balance may only read the mass of an object to the nearest tenth of a gram, many digital scales can measure the mass of an object up to the nearest thousandth of a gram

Accuracy and Precision of a Measurement

Science is based on observation and experiment - that is, on measurements. Accuracy is how close a measurement is to the correct value for that measurement. For example, let us say that you are measuring the length of standard computer paper. The packaging in which you purchased the paper states that it is 11.0 inches long. You measure the length of the paper three times and obtain the following measurements: 11.1 in., 11.2 in., and 10.9 in. These measurements are quite accurate because they are very close to the correct value of 11.0 inches. In contrast, if you had obtained a measurement of 12 inches, your measurement would not be very accurate.

The precision of a measurement system refers to how close the agreement is between repeated measurements (which are repeated under the same conditions). Consider the example of the paper measurements. The precision of the measurements refers to the spread of the measured values. One way to analyze the precision of the measurements would be to determine the range, or difference, between the lowest and the highest measured values. In that case, the lowest value was 10.9 in. and the highest value was 11.2 in. Thus, the measured values deviated from each other by at most 0.3 in. These measurements were relatively precise because they did not vary too much in value. However, if the measured values had been 10.9, 11.1, and 11.9, then the measurements would not be very precise because there would be significant variation from one measurement to another.

The measurements in the paper example are both accurate and precise, but in some cases, measurements are accurate but not precise, or they are precise but not accurate. Let us consider an example of a GPS system that is attempting to locate the position of a restaurant in a city. Think of the restaurant location as existing at the center of a bull's-eye target, and think of each GPS attempt to locate the restaurant as a black dot. In Figure 1.24, you can see that the GPS measurements are spread out far apart from each other, but they are all relatively close to the actual location of the restaurant at the center of the target. This indicates a low precision, high accuracy measuring system. However, in Figure 1.25, the GPS measurements are concentrated quite closely to one another, but they are far away from the target location. This indicates a high precision, low accuracy measuring system.

A pattern similar to a dart board with few concentric circles shown in white color on a red background. In the innermost circle, there are four black points on the circumference showing the positions of a restaurant. They are far apart from each other

Figure 1.24 A GPS system attempts to locate a restaurant at the center of the bull's-eye. The black dots represent each attempt to pinpoint the location of the restaurant. The dots are spread out quite far apart from one another, indicating low precision, but they are each rather close to the actual location of the restaurant, indicating high accuracy.

A pattern similar to a dart board with a few concentric circles shown in white color on a red background. Near the outermost white circles there are four black points showing the positions of a restaurant. The black points are very close to each other

Figure 1.25 In this figure, the dots are concentrated rather closely to one another, indicating high precision, but they are rather far away from the actual location of the restaurant, indicating low accuracy.

Accuracy, Precision, and Uncertainty

The degree of accuracy and precision of a measuring system are related to the uncertainty in the measurements. Uncertainty is a quantitative measure of how much your measured values deviate from a standard or expected value. If your measurements are not very accurate or precise, then the uncertainty of your values will be very high. In more general terms, uncertainty can be thought of as a disclaimer for your measured values.

For example, if someone asked you to provide the mileage on your car, you might say that it is 45,000 miles, plus or minus 500 miles. The plus or minus amount is the uncertainty in your value. That is, you are indicating that the actual mileage of your car might be as low as 44,500 miles or as high as 45,500 miles, or anywhere in between. All measurements contain some amount of uncertainty. In our example of measuring the length of the paper, we might say that the length of the paper is 11 in., plus or minus 0.2 in. The uncertainty in a measurement, $A$ , is often denoted as $\delta A$ ("delta $A$ "), so the measurement result would be recorded as $A \pm \delta A$ . In our paper example, the length of the paper could be expressed as 11 in. $\pm 0.2$ .

The factors contributing to uncertainty in a measurement include:

Limitations of the measuring device,
The skill of the person making the measurement,
Irregularities in the object being measured,
Any other factors that affect the outcome (highly dependent on the situation).

In our example, such factors contributing to the uncertainty could be the following: the smallest division on the ruler is 0.1 in., the person using the ruler has bad eyesight, or one side of the paper is slightly longer than the other. At any rate, the uncertainty in a measurement must be based on a careful consideration of all the factors that might contribute and their possible effects.

Making Connections: Real-World Connections —Fevers or Chills?

Uncertainty is a critical piece of information, both in physics and in many other real-world applications. Imagine you are caring for a sick child. You suspect the child has a fever, so you check his or her temperature with a thermometer.

What if the uncertainty of the thermometer were $3.0^{\circ} \mathrm{C}$ ? If the child's temperature reading was $37.0^{\circ} \mathrm{C}$ (which is normal body temperature), the "true" temperature could be anywhere from a hypothermic $34.0^{\circ} \mathrm{C}$ to a dangerously high $40.0^{\circ} \mathrm{C}$ . A thermometer with an uncertainty of $3.0^{\circ} \mathrm{C}$ would be useless.

Percent Uncertainty

One method of expressing uncertainty is as a percent of the measured value. If a measurement $A$ is expressed with uncertainty, $\delta A$ , the percent uncertainty (\%unc) is defined to be

$\% \text { unc }=\frac{\delta A}{A} \times 100 \%.$

Uncertainties in Calculations

There is an uncertainty in anything calculated from measured quantities. For example, the area of a floor calculated from measurements of its length and width has an uncertainty because the length and width have uncertainties. How big is the uncertainty in something you calculate by multiplication or division? If the measurements going into the calculation have small uncertainties (a few percent or less), then the method of adding percents can be used for multiplication or division.

This method says that the percent uncertainty in a quantity calculated by multiplication or division is the sum of the percent uncertainties in the items used to make the calculation. For example, if a floor has a length of $4.00 \mathrm{~m}$ and a width of $3.00 \mathrm{~m}$ , with uncertainties of $2 \%$ and $1 \%$ , respectively, then the area of the floor is $12.0 \mathrm{~m}^{2}$ and has an uncertainty of $3 \%$ . (Expressed as an area this is $0.36 \mathrm{~m}^{2}$ , which we round to $0.4 \mathrm{~m}^{2}$ since the area of the floor is given to a tenth of a square meter).

Source: Rice University, https://openstax.org/books/college-physics/pages/1-3-accuracy-precision-and-significant-figures
This work is licensed under a Creative Commons Attribution 4.0 License.