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.
Two common errors
Zero error
The instrument reads something when it should read zero.
Press the tare button, or subtract the zero error from every reading.
Parallax error
Reading a scale from the wrong angle.
Put your eye level with the mark and look straight at the scale.
Worked example
An empty balance shows 0.2 g.
A stone shows 15.6 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.
Name the error, for example parallax error when reading the measuring cylinder.