PSA: Accuracy and Precision


A pet peeve of mine – using the words accuracy and precision incorrectly, and failing to make a distinction between the two.

Accuracy;
“degree of conformity of a measure to a standard or a true value”
accuracy. (2008). In Merriam-Webster Online Dictionary.
Retrieved October 6, 2008, from http://www.merriam-webster.com/dictionary/accuracy

Precision;
“the degree of refinement with which an operation is performed or a measurement stated”
precision. (2008). In Merriam-Webster Online Dictionary.
Retrieved October 6, 2008, from http://www.merriam-webster.com/dictionary/precision

In watches, accuracy is how well the watch keeps time and reflects the time according to an accepted standard. That is to say, how many seconds the watch gains or loses in a given period and how close it is to the Observatory clock. A Movado with no markings may be as accurate as an Omega with a similar movement, if they gain or lose time similarly (so many seconds a week, for example) and are set correctly.

Precision is the markings on the face, or the decimal places in the digital read out, with smaller intervals of time measured reflecting greater precision. John’s personal favorite, the Alpha GMT has the customary degree of precision, but I invite you to consider John’s observations about its accuracy. I have had similar experiences with Vostok, which represents everything I’ve learned to expect from Russian-made machinery.

So a Movado can be accurate, but not precise, while a Faux-lex, $15 knock off of a good watch, is precise without being accurate. Look at a watch with only hour markings, and you can estimate the minutes +/-1, so you can say that it is about 2:17, and if the atomic clock at the National Observatory says anything between 2:16 and 2:18, the watch is accurate but not precise. Look at a watch that displays seconds, and if it reads 2:17:31, and the time is 2:13:08, it is precise, but less accurate.

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  • Cliff

    But is not this discussion like one about “significant figures?” You can only compare two numbers to the one with the least decimal places. For example 2.5 * 2.3 = 5.8, not 5.75.

    Therefore in the example above, would not the Movado and the Faux-lex both have the same accuracy, as you can only compare to the nearest minute?

  • Steven

    Therefore in the example above, would not the Movado and the Faux-lex both have the same accuracy, as you can only compare to the nearest minute?

    Extending this premise to its logical conclusion, your Movado is no more accurate than my calendar, if they both tell me it is December 3.

  • E.M. Van Court

    Cliff, accuracy is how consistantly the movement makes the hands go and how well it is calibrated to a standard. Precision is about the markings, like significant figures you mention, so no, the knock off could gain and loss minutes a day and be precise in its markings but hideously inaccurate. The Movado uses an excellent, well calibrated movement, but the lack of markings gives it a relatively low degree of precision.

    Steven, the calander tells you it is Dec. 3, the Movado tells you it is 1:40-ish. Accuracy in the absence of a mechanism (like a calendar) is only about correctness of the printing where a watch is still concerned with mechanical calibration as well as design, but the Movado is much more precise (down to a fraction of an hour).

    Oh, and it’s not ‘my Movado’… The edgey cool of the Museum face has been diluted by their marketing people.

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