Thursday, February 14, 2008

Accuracy vs. Precision
Precision: The ability of a measurement to be reproduced consistently. The number of significant digits to which a value may be measured reliably. http://en.wiktionary.org/wiki/precision

Accuracy: The degree of conformity of a measured or calculated quantity to its actual (true) value. http://en.wikipedia.org/wiki/Accuracy

Accuracy is closely related to precision, also called reproducibility or repeatability, the degree to which further measurements or calculations show the same or similar results. The results of calculations or a measurement can be accurate but not precise; precise but not accurate; neither; or both. http://en.wikipedia.org/wiki/Accuracy

If the average tends to the true value as the number of data points tends to infinity, it can be accurate even if not precise. For example, the Iosipescu test is not precise for G12 for heterogeneous samples because the test section is small, and thus one specimen has more fiber in the test section, other more matrix. But, it is accurate is you do many tests. The torsion test is precise for G12 even for heterogeneous tests because the test section is the whole cross section of the specimen.

See also http://en.wikipedia.org/wiki/Accuracy#Accuracy_vs_precision_-_the_target_analogy

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