Chemistry: Precision vs Accuracy
Accuracy is defined as the closeness of measured values to the actual or accepted value.
Precision is defined as the closeness among measured values of some quantity.
If the standard value of length of an object is 50 cm. Two students recorded its length five times.
1: The first student recorded the length as 52, 47, 50, 51 and 49 cm.
2: The second student recorded measurements as 44, 45, 45, 44 and 44 cm.
In this case the measurements recorded by first student are although more accurate but less precise because they are closer to the actual measured value. While the measurements of second student are more precise but less accurate because they are closer to each other irrespective of their closeness to the actual value.
The accuracy of a measurement is the direct measure of fractional or percentage uncertainty in that measurement. More accurate reading has less fractional or percentage uncertainty.
The precision of a measurement depends upon the instrument that is being used. Precision is a direct measure of least count (absolute uncertainty) of that instrument. A reading is said to be more precise if the least count or absolute uncertainty of instrument is less.
Fractional uncertainty = absolute uncertainty ÷ measured value
Percentage uncertainty = absolute uncertainty × 100