[time-nuts] What is "accuracy"? (newbie timenut, hi folks!)

[James Flynn]

Belinda,

It is often good to ask such questions.  So, please do not apologize. 

Accuracy is defined as how close a claimed parameter of a device or 
system (frequency, time, etc.) is to a standard. This claim has to be 
supported by measurements made to a certain precision, (there's that 
other word) which is the quantifying of the possible uncertainty in the 
measurement.

As with any parameter, the CONDITIONS under which the claim is made can 
vary from creator/manufacturer to creator/manufacturer.

For example, I can claim my clock is accurate to 1 part in a trillion 
(1e-12) UNDER THE CONDITION that the operating temperature is kept 
between 10C and 30C.  Outside of those limits, the accuracy is outside 
this range. 

To make this claim, however, I would have to also make measurements to a 
PRECISION that is much smaller than the ACCURACY I claim.  

An example of this is measuring the accuracy of my clock's 1 second 
pulse.  That is, how close it is to "on time" as put out by my country's 
bureau of standards.  If I claim it is ACCURATE to +/- one millisecond, 
but my measurement device can only measure in tenths of a second, i.e. 
its PRECISION is in 0.1 seconds, my claim is on shaky ground.  On the 
other hand, if the precision of my measurements is to one microsecond, 
then my claim has merit.

Also I can claim that my clock has this accuracy only 99% of the time 
under these conditions. Now we have a more fuzzy boundary.   

So, you see that while accuracy itself is well defined, the conditions 
under which the claim is made can vary quite a bit.

You are correct that "precision" and "accuracy" are often confused and 
misused. 

It is good to clarify these two important principles. So, do lots of 
reading and research.  In a business focused on "precision" and 
"accuracy", it is important to be precise and accurate in what you are 
thinking/writing/speaking about.