Accuracy and Precision

Accuracy: A measure of how close your measured value is to the actual value.

Precision: A measure of how close measured values are to each other.

Random Error: Errors that occur in an infrequent or random manner during a series of measurements, due to non-repetative causes. Examples include spilling, an error in recording a measurement, or an error in reading an instrument that happens once.

Systematic Error: Errors that are repeated throughout a series of measurements. Examples include errors due to a poorly calibrated instrument, or a misunderstanding of how to read an instrument that is reflected in each measured value.

1. A beaker is known to contain 47.3 mL of dichloromethane (a common solvent in organic labs). Abby measures the volume three times and obtains values of 46 mL, 48 mL and 47 mL. Billy measures the volume three times and obtains values of 40 mL, 41 mL and 47 mL. Who is more accurate? Who is more precise?

2. Candy and Dave each measure the mass of a piece of filter paper. Candy takes five measurements, with values of 1.13g, 1.11g, 1.10g, 1.15 g and 1.17g. Dave takes 3 measurements, with values of 1.00g, 1.67g, and 1.34 g. The actual mass of the filter paper is 1.34 g. Who is more accurate? Who is more precise?

3. Erik and Frank are each asked to determine the length of a paperclip. Erik is given a small ruler with each millimeter marked. Frank is given a meter stick with no additional markings. Who would you expect to be more accurate in their measurements? Why?

1. Abby is more... hey, no cheating! Are you sure you've finished the problem?

1. Abby is more precise and more accurate.

2. Candy is more precise, Dave is more accurate.

3. We would expect Erik to be more accurate, since his ruler has smaller gradations and he can take more accurate measurements.