Before any evaluation of measured values of a measured variable, a test should be conducted as to whether the measurements have been taken under the same test conditions and independently of one another. To that end, it is sufficient to issue two limit cases from a variety of possible test conditions.
Repeat conditions:
are present if the same observer performs measurements according to a defined measuring procedure on the same measurement object under the same test conditions (same measuring device, same laboratory) several times in short time intervals. The standard deviation under repeat conditions is called the repeatability standard deviation.
Under repeat conditions, the same systematic deviations occur with each measurement. They can therefore not be defined outside of the repeat measurement series.
Conditions of comparison:
are present if different observers take measurements at different times according to a defined measurement procedure on the same measurement object under different test conditions (different measuring devices, different test locations or laboratories).
The standard deviation under conditions of comparison is called the standard deviation of reproducibility.
Under conditions of comparison, when comparing the measured values from different laboratories, systematic deviations may occur, which cannot be determined without further effort in each individual laboratory.
Example: In the case of several standardised measurement procedures in the area of mineral oil testing, the standard deviation of reproducibility is around twice as large as the repeatability standard deviation (DIN 1319 Part 3).