Measurement: Precision versus Accuracy

Accuracy and precision are two indicators of the validity of measurements. Accuracy is the agreement of a measurement is with an actual value (how close it is to what it is supposed to be). Unfortunately, if something is truly worth measuring, we don’t know its actual value. Accuracy is important in the chemistry lab during exercises in which the lab instructor knows what the value actually is and the students try to get a result as close as possible to this ‘true’ value. Scientists routinely calibrate their instruments (check them for for accuracy) by using them to measure a standard for which the values are well known. If the instrument’s readings match the accepted values for the standard, it is probably working correctly.

Precision, on the other hand, is an indication of how close several measurements are to one another. Systematic errors tend to have an additive effect. If a student has poor laboratory techniques, his or her measurements will tend to fluctuate more than those of another student with good techniques, and the average (mean) will have a high deviation. Laboratory instructors will tend to have students perform measurements several times then calculate an mean and average deviation from the mean. If the instructor knows what the result should be, the student’s mean indicates the accuracy and the average deviation from the mean indicates the precision.

To find the mean of a set of experimental results, add the results and divide by the number of trials. To find the average deviation from the mean, subtract each result from the mean, then take the average of the absolute values of these differences.

Concept Check: Two students perform an lab experiment to find the density of an unknown metal. Their results are in the table below. Which student should receive a higher grade and why? The unknown metal is nickel which has a density of 8.91 grams per cubic centimeter.

April			May
Trial 1	Trial 2	Trial 3	Trial 1	Trial 2	Trial 3
8.77 g/cm3	8.85 g/cm3	9.04 g/cm3	8.86 g/cm3	8.89 g/cm3	8.90 g/cm3
Mean = 8.89 g/cm3			Mean = 8.88 g/cm3
Average Deviation = 0.10 g/cm3			Average Deviation = 0.02 g/cm3

Answer: May should get a better grade. While April's mean is slightly closer to the actual value, her results had poor precision.

This site was last updated 08/01/06