Introduction to ASTM test method - Explanation of Terms
Accuracy - The accuracy of a test is a measure of how close the test result will be to the true value of the property being measured. As such the accuracy can be expressed as the bias between the test result and the true value. However, the absolute accuracy can only be established if the true value is known.
AFNOR - Association Francaise de Normalisation (Paris).
ASTM - American Society for Testing and Materials (U.S.A.).
DIN - Deutsche Institut Fur Normung (Germany).
IP - Institute of Petroleum (U.K.); now called as Energy Institute.
ISO - International Organization for Standardization (Switzerland).
JIS - Japan Industrial Standards (Tokyo).
Precision - The precision of a test method is defined in terms of the variability between test results obtained on the same material, using a specific test method. The precision of a test is usually unrelated to its accuracy. The results may be precise, but not necessarily accurate. Figures 1 to 4 depict in a bull's eye analogy the relation between precision and accuracy. Ideal condition would be most precise and most accurate results. Precision is expressed as repeatability and reproducibility.
Repeatability - The "within-laboratory precision" refers to the precision of a test method when the results are obtained by the same operator in the same laboratory using the same apparatus.
Repeatability or repeatability interval of a test (indicated with the letter "r") is defined as the maximum permissible difference due to test error between two results obtained on the same material in the same laboratory.
r = 2.77 x standard deviation of test
Most commonly this repeatability interval r is statistically defined at the 95 % probability level, meaning that, even in normal conditions, differences between two test results are unlikely to exceed this repeatability interval more than five times in a hundred.
Reproducibility - The "between-laboratory precision" is defined in terms of the variability between test results obtained on the aliquots of the same homogeneous material in different laboratories using the same test method.
The term reproducibility or reproducibility interval (designated as "R") is completely analogous to the term repeatability. Only in this case, it is the maximum permissible difference between two results obtained on the same material but now in different laboratories. The statistical definition of reproducibility is along the same lines as above. Therefore differences between two or more laboratories are unlikely to exceed the reproducibility interval more than five times in a hundred.
R = 2.77 x standard deviation of test
Reproducibility is generally higher than repeatability by a factor of 2 to 4.
The repeatability and reproducibility values have very important implications in today's quality conscious market. As the demand for clear product specifications, and hence control over product consistency grows, it is meaningless to establish product specifications that are more restrictive than the reproducibility/repeatability values of the specification test methods.
