Accuracy Definition in Science

Accuracy refers to the correctness of a single measurement. Accuracy is determined by comparing the measurement against the true or accepted value. An accurate measurement is close to the true value, like hitting the center of a bullseye.

Contrast this with precision, which reflects how well a series of measurements agree with each other, whether or not any of them are close to the true value. Precision can often be adjusted using calibration to yield values that are both accurate and precise.

Scientists often report percent error of a measurement, which expresses how far a measured value is from the true value.

Examples of Accuracy in Measurements

For example, if you measure a cube that is known to be 10.0 cm across and your values are 9.0 cm, 8.8 cm, and 11.2 cm, these values are more accurate than if you had gotten values of 11.5 cm, 11.6 cm, and 11.6 cm (which are more precise).

Different types of glassware used in the lab are inherently different in their level of accuracy. If you use an unmarked flask to try to obtain 1 liter of liquid, you're likely not going to be very accurate. If you use a 1-liter beaker, you'll probably be accurate within several milliliters. If you use a volumetric flask, the accuracy of the measurement may be within a milliliter or two. Accurate measuring tools, such as a volumetric flask, are usually labeled so a scientist knows what level of accuracy to expect from the measurement.

For another example, consider mass measurement. If you measure mass on a Mettler scale, you can expect accuracy within a fraction of a gram (depending on how well the scale is calibrated). If you use a home scale to measure mass, you usually need to tare the scale (zero it) to calibrate it and even then will only get an inaccurate mass measurement. For a scale used to measure weight, for example, the value could be off by half a pound or more, plus the accuracy of the scale may change depending on where you are in the instrument's range. A person weighing close to 125 lbs might get a more accurate measurement than a baby weighing 12 lbs.

In other cases, accuracy reflects how close a value is to a standard. A standard is an accepted value. A chemist might prepare a standard solution to use as a reference. There are also standards for units of measurement, such as the meter, liter, and kilogram. The atomic clock is a type of standard used to determine the accuracy of time measurements.