Repeating readings reduces the effect of random error, but it cannot fix systematic error, because every repeat is shifted by the same amount. To improve a biased method you must find and correct the source of the bias.
This lesson is part of investigations and evidence. It follows from improving a measurement. The data is invented.
Why does averaging not help?
Averaging works when errors point in different directions and partly cancel. A systematic error points in one direction every time, so there is nothing to cancel. The mean simply carries the same shift.
Worked example
A balance has an unnoticed zero error and reads 0.5 g too high. A student measures a sample three times: 12.6 g, 12.5 g and 12.6 g. The sample’s true mass is 12.1 g.
Step 1, find the mean. 12.6 + 12.5 + 12.6 = 37.7, and 37.7 ÷ 3 = 12.57, so about 12.6 g.
Step 2, compare with the truth. 12.6 − 12.1 = 0.5 g too high, exactly the zero error.
Step 3, describe the readings. They agree closely (range 0.1 g), so they are precise, but they are not accurate.
Step 4, give the fix. Check the empty balance reads zero, or check it with a known mass, and correct before measuring. More repeats on the faulty balance would still give about 12.6 g.
The mistake to watch for
Mistaken answer: “The readings are close together, so the mean must be correct.”
Agreement between readings shows low scatter only. The correction is to also ask whether anything could shift every reading the same way, such as an unchecked zero or a method that always loses a little of the sample.
Check yourself
1. Four readings from a faulty balance are 8.4 g, 8.4 g, 8.5 g and 8.3 g. What is the mean, and does averaging remove a 0.5 g zero error?
Show answer
8.4 + 8.4 + 8.5 + 8.3 = 33.6, and 33.6 ÷ 4 = 8.4 g. No, the mean still contains the 0.5 g error.
2. A student says “repeating three times fixed the bias.” Reply in one sentence.
Show answer
Repeating reduces random scatter but leaves a consistent shift unchanged, so the cause of the shift must be found and corrected.
3. Which describes precision: “close to the true value” or “close to each other”?
Show answer
“Close to each other.” Accuracy is closeness to the true value.
Where this leads next
Next, compare an experimental conclusion with an alternative. The scientific investigation critic uses this exact case, repeating a biased reading, as a check on your reasoning.
A teacher in online one-to-one Combined Science tuition can build more cases like this from your own class practicals.