In short
An error in a measurement is the difference between the measured value and the true value. Systematic errors push every reading the same way, and random errors scatter the readings.
Systematic errors come from a faulty instrument or method, such as a zero error, and repeating the experiment does not remove them. Random errors vary from reading to reading, and taking the mean of many readings reduces them. The absolute error is the size of the difference, and the percentage error is . When you add or subtract two quantities, their absolute errors add.
A length is measured as and another as . Their sum is , because absolute errors add.
Also searched as: adding errors, precision error meaning, random and systematic errors.
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