Error is the deviation of the obtained result from the true, expected value or the estimated central value. Errors are expressed in absolute or relative terms.

Absolute error in a measurement is the numerical difference from the true or central value. Relative error is the ratio between absolute error and the true or central value, expressed as a percentage.

Errors can be classified by source, magnitude, and sign. There are three types of errors: systematic, random, and gross.

Systematic or determinate errors emerge from known sources and are reproducible during replicate measurements. Defective equipment and experiment design flaws are familiar sources of these errors. These errors can be minimized by employing standard reference materials, independent analysis, or varying the sample size.

Random errors or indeterminate errors are difficult to reproduce by repeating measurements. These errors originate from uncontrolled variables like electronic noise in the circuit of an electrical instrument and irregular changes in the heat loss rate from a solar collector due to changes in the wind.

Gross errors are caused by human mistakes. The magnitude of these errors is often high. The origin of such errors is entirely based on the observer.