A systematic offset shifts every reading by the same amount. A supplied calibration reading, such as the value shown with nothing measured, tells you the size of the shift so that you can correct the data.
This lesson is part of uncertainty and practical-data reasoning. It extends the zero-error idea in reading measuring instruments and uncertainty.
What is the method?
Follow three steps.
- Find the calibration reading: the value the instrument shows for zero or a known standard.
- Calculate the offset: observed calibration reading minus the true value.
- Correct every measurement: corrected value = observed value − offset.
Worked examples
Balance. An empty pan reads 0.4 g. Three readings of an object are 12.7 g, 12.8 g and 12.7 g. The offset is +0.4 g, so the corrected readings are 12.3 g, 12.4 g and 12.3 g, and the mean is 12.3 g.
Ammeter. With no current, the ammeter shows 0.05 A. A reading of 0.85 A becomes 0.85 − 0.05 = 0.80 A.
Thermometer in melting ice. The true value is 0 °C but the thermometer reads 1.5 °C, so the offset is +1.5 °C. A reading of 61.5 °C becomes 61.5 − 1.5 = 60.0 °C.
Why does repeating not fix it?
In the balance example, the three readings agree closely, so the spread is small. They are still all 0.4 g too high. Random scatter shows up as spread, while a systematic offset hides inside agreeing readings.
| Type of error | Shows up as | Reduced by |
|---|---|---|
| Random | Spread between repeats | Averaging repeated readings |
| Systematic | All readings shifted together | Calibration and correction |
The mistake that costs marks
The common slip is to say “repeat the readings and take the average” as a remedy for a zero error. That answer does not remove a systematic offset, so it earns no mark.
A better answer is: “Record the reading with nothing measured, then subtract it from every reading.” Use the word offset or zero error in your answer.
Check yourself
A newton meter reads 0.3 N when nothing is hung from it. A student measures 2.8 N and 3.1 N for two loads. Find the corrected readings and explain why the student cannot fix the problem by repeating the readings.
Answer
Offset = +0.3 N.
Corrected readings: 2.8 − 0.3 = 2.5 N and 3.1 − 0.3 = 2.8 N.
Repeating the readings gives the same shifted values every time, because the error is the same in every measurement. Only subtracting the zero reading corrects it.
What to study next
Finish this section with stating a conclusion at a precision justified by the data. The units and significant-figure checker helps with presenting results.
If you want a teacher to give you calibration cases to practise, see online one-to-one Physics tuition. The mistake log and paper-error review tool helps you track what you miss.