In case of repeated observation of any parameter, usually it has been found to have variations, however small, in the resulting measurement. Moreover, there is nothing definite in the amount of variation i.e., variations are random in nature. Thus, a measurement usually differs from its true value . The difference between a measured and its true value is called the measurement error. Thus, if x is a given measurement and x _t is the true value, then the error e is given by

e = x - x _t

error = measured value – true value.

If an estimated value of x_t is usually known and is denoted by x¹. Then, an estimate of error for a measurement value x of the parameter is obtained as

e¹ = x - x¹

However, correction is the term more popularly being used to define the magnitude of error but opposite in sign. Thus, rearranging the error relation,

correction = (-e¹) = x¹ - x

or, correction = (estimated / designated) true value - measured value.

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