Answer
In bunker fuel testing, Repeatability (r) and Reproducibility (R) describe the expected variation between laboratory test results. They are statistical precision limits used when interpreting fuel-analysis results under ISO 4259 (which governs the precision of test methods cited in ISO 8217).
Repeatability — rThis is the expected maximum difference between two test results on the same sample, when testing is done:
- in the same laboratory
- by the same operator
- using the same equipment
- using the same test method
- within a short time
If the difference between the two results exceeds r, the results are considered suspicious and the test should normally be investigated/repeated.
Easy memory:
Repeatability = Same lab, same man, same machine.
This is the expected maximum difference between two independent results on the same material, when testing is done:
- in different laboratories
- by different operators
- using different equipment
- but using the same specified test method
Because more variables are involved:
R>r normally.
Why important in bunker disputes
Suppose the supplier's laboratory says:
Sulphur = 0.49% m/m
and the ship/independent laboratory obtains:
Sulphur = 0.52% m/m.
We should not immediately conclude that one laboratory is wrong. The permitted test-method precision and the applicable reproducibility, R, have to be considered when comparing independent laboratory results or resolving a specification dispute. ISO 4259-2 specifically covers application of repeatability/reproducibility and dispute procedures.