If at least one component of a trial result is randomly distributed, then a statistical model can be formulated, e.g. as follows:
xi = m + ei ,
xi is the value at observation i
m the expectation value
ei the random deviation from the expectation value at observation i.
The component ei is assumed, for example, to be Normally distributed with expectation value = 0 and standard deviation = s.
The above can be a model of repeated measurements of a test object under repeatability conditions (see ISO 5725, part 1), that is, measurements made during a short period of time at the same laboratory and operator, instrument and other equipment. If one or more of these conditions are varied, then often the scatter will increase. If measurements were to be made on different days, it is possible that the expectation value varies from day to day. If one or more of these conditions vary, then the total scatter is often going to increase. This situation could be described with the model
xij = m + di + eij ,
xij is the value of observation j on day i
m the overall expectation value
di the difference between the expectation value of day i and the overall expectation value
eij the difference between observation j day i and the expectation value of day i.
Here it is assumed that both components di and eij vary randomly with expectation values = 0 and standard deviations sd and s respectively. If more identifiable conditions are varied, then additional error components can be introduced in the model. In order to estimate the standard deviations of these components and to test whether they are non-zero, a statistical method is used called variance analysis (ISO5725, parts 2 and 3).