Definition
Measurement error
The signed difference between a measured value and a trusted reference value.
What it means
A measuring tool gives a reading, but the reading may not land exactly on a trusted comparison value. Subtract the reference from the reading. A positive error means the reading is high; a negative error means it is low.
Remember it
Reading minus reference: the sign points high or low.
Example
A ruler reads 12.0 cm for a length whose reference value is 12.4 cm. The measurement error is 12.0 − 12.4 = −0.4 cm, so the reading is 0.4 cm low.
Watch for
Measurement uncertainty is not another name for measurement error. Error compares one result with a reference; uncertainty describes the spread of values that could reasonably fit the measurement.
Common mix-up
The size of the error is 0.4 cm, but the signed error is −0.4 cm because the reading is low.
Exact meaning
Measurement error is the measured value minus a reference value for the same quantity. Its sign shows whether the measurement is above or below that reference.