The Percentage error is a measurement of the discrepancy (or deviation) between an observed and a true, or accepted value. While measuring data, the result often varies from the true value. This could be because of an error such as a wrong input or empty field. 

Error can arise due to many different reasons that are often caused by human negligence. However, sometimes can also be due to estimations and limitations of devices used for the purpose of computation and measurement. 

Regardless of it, in cases such as these with some sort of incorrect input, it can be valuable to calculate the percentage error (also known as the margin of error). 

The computation process of percentage error involves the use of the absolute error, which is simply the difference between the observed and the true value. 

The absolute error is now divided by the true value, resulting in the relative error, which is then multiplied by 100 to obtain the percentage error. 

Refer to the equations below for clarification:

Absolute error = |Vtrue – Vobserved

Relative error = |Vtrue – Vobserved|Vtrue

Percentage error = |Vtrue – Vobserved|Vtruex 100

The equations above are based on the assumption that true values are known. True values are often unknown, and under these situations, standard deviation is one way to represent the error. 

You can also read about Cronbach’s Alpha to know more about reliability tests.