PSA: Accuracy and Precision
“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
“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.