About measurement errors
Mention must also be made of measurement
errors. Measurement system errors can be divided into two categories, systematic and random
errors. Various mechanisms exist for reducing these two types of error.
A characteristic feature of all systematic errors is that they produce
errors which are consistently on the same side of the true value, i.e.
either all the errors are positive or they are all negative. Systematic
errors arise from many causes.
These include system disturbance due to measurement, environmental changes
(modifying inputs) and drift in instrument characteristics. Large errors
due to instrument characteristic drift are avoided by recalibrating
instruments at suitable intervals. In the case of other
sources of systematic error, a good measurement technician can largely
eliminate errors by calculating their effect and correcting the
measurements. This is done automatically by using intelligent instruments.
Random errors are in many ways easier to deal with because they consist
generally of small perturbations of the measurement either side of the
correct value, i.e. positive errors and negative errors occur in
approximately equal numbers for a series of measurements made of the same
quantity. Therefore, random errors can be largely eliminated by averaging
a few measurements of the same quantity. Unfortunately, averaging a number
of measurements cannot be guaranteed to produce a value close to the true
one because random errors occasionally cause large perturbations from the
true value. Thus, it is necessary to describe measurements subject to
random errors in probabilistic terms, typically that there is a 95%
probability that the measurement error is within boundaries of ± 1% from
the true value.
Principles of Measurement