A measurement is a number plus a unit, such as 5.00 cm, 20 s or 100 g.
Accuracy vs precision
Accuracy is how close a reading is to the true value. Precision is how close repeated readings are to each other.
Four targets showing accuracy and precision Four targets. Top left: dots tight in the centre, accurate and precise. Top right: dots tight but away from the centre, precise but not accurate. Bottom left: dots spread around the centre, accurate but not precise. Bottom right: dots spread and off the centre, neither.
Accurate, precise Close together, on target
Precise, not accurate Close together, off target
Accurate, not precise Scattered around the target
Neither Scattered and off target
Two common errors
Zero error The instrument reads something when it should read zero.
Parallax error Reading a scale from the wrong angle.
Worked example
An empty balance shows 0.2 g.
true mass = 15.6 − 0.2 = 15.4 g
Random errors are unpredictable, so repeat the reading and take the average. Systematic errors are the same every time, such as a zero error, so correct each reading.
Gentle reminder "Human error" scores nothing by itself.