Resolution and Accuracy

Resolution is the smallest change in the quantity being measured that gives a perceptible change in the reading of an instrument. A metre rule marked in millimetres has a resolution of 1 mm, a digital voltmeter showing 3.46 V has a resolution of 0.01 V, and a micrometer has a resolution of 0.01 mm.A finer resolution lets you detect smaller changes, but it does not guarantee a correct answer. Choose an instrument whose resolution is small compared with the quantity: a metre rule is fine for a 50 cm length, but for the 0.5 mm diameter of a wire a micrometer is used instead.Accuracy describes how close a measurement is to the true value. A measurement is accurate if it is close to the accepted value. Accuracy is reduced by systematic errors, such as a zero error or a badly calibrated instrument, which shift every reading in the same direction.Resolution, precision and accuracy are separate ideas. A digital thermometer with a resolution of 0.1 °C that reads 2.0 °C too high gives high-resolution, precise readings that are all inaccurate: when it shows 22.4 °C the true temperature is 20.4 °C. Calibrating an instrument against a known value (for example, checking a thermometer in melting ice at 0 °C) improves accuracy.Example: Two students measure g. Student A gets 9.6 m s⁻² and student B gets 9.9 m s⁻². The accepted value is 9.81 m s⁻². Student A is 0.21 m s⁻² away and student B only 0.09 m s⁻² away, so student B's value is more accurate.