As your cleverest professors might be fond of saying: “A measurement can be accurate without being precise; a measurement can be precise without being accurate.” A simple demonstration of this distinction: We can refer to a wrapped collection of hay as a bale (an accurate measurement) without precisely counting its strands; we can scatter the hay and number the strands (a precise measurement) but not accurately call it a bale. More to the point, we cannot claim that a particular event occurred “precisely 20,000 years ago” or that a particular ore reserve weighs “precisely 1 million tonnes”; by definition, such values are measured coarsely rather than exactly. In relation to the weather, we would properly refer to an accurate (true) forecast, but a precise (exact) temperature.

“Accuracy” denotes how closely a measurement approaches its true value. An accurate measure, then, is one that conforms well to an implied or stated benchmark:

The accuracy of the test results was verified by running 50 of the samples a second time.

This particular scale is accurate to the nearest kilogram.

“Precise” means marked by a high degree of exactitude:

One pint is precisely 568.245 milliliters.

In the simplest terms, accuracy is about conformity to truth or fact, while precision is about exactness.