Tools & Microfiber2 min read

Accuracy and Precision in Coating Tests: Repeatable Can Still Be Wrong

Understand how repeatable readings can remain biased, and use suitable references before claiming accuracy in a coating comparison.

Updated · NANOLAB Knowledge

A measurement process can produce closely grouped readings while remaining systematically displaced from a suitable reference. Precision concerns agreement among repeated results under stated conditions; accuracy concerns closeness to the relevant true or reference value. Good repeatability alone does not establish accuracy.

Use a concrete comparison

Imagine a reference associated with a value of 100 units. One illustrative instrument repeatedly reads near 105. Another produces readings spread around 100. The first appears more consistent, but its bias must be investigated. The second may have an unacceptable spread even though its average is close.

Neither pattern should be reduced to “good” without considering the required tolerance and measurement uncertainty. Averages can hide both systematic error and inconvenient variation.

Check references and repeated use

Follow the instrument's verification procedure with suitable reference material. Then examine repeatability under the actual inspection conditions, including repositioning if the real task involves repositioning. Keep reference checks separate from readings on variable customer surfaces.

If a reference check fails, review results collected since the last satisfactory check according to the quality procedure. Repeating the same incorrect reading several times does not solve the problem.

Describe the claim precisely

Avoid saying a process is “accurate to two decimals” merely because the display shows two decimals. State the method, units, reference information and relevant uncertainty or specification. Use the supplier's terminology carefully rather than substituting marketing language for metrology.

For a NANOLAB product comparison, ensure that a claimed difference is larger than the relevant measurement limitations and is reproduced across appropriate samples. If the data do not support that conclusion, report the limitation and use a better-designed test. A disciplined measurement process is more persuasive than a table of highly precise-looking numbers with no reference checks.

Related guidance

Technical references: NIST: Measurement terminology.

For product-specific application instructions, consult NANOLAB technical documentation or ask about your surface and intended use.