[LEAPSECS] Accuracy and Precision

Finkleman, Dave dfinkleman at agi.com
Wed Jan 12 12:33:18 EST 2011

These terms have appeared in recent exchanges. Keeping the distinction
clear is one of my continuing quests. Perhaps I have it wrong, too. I
am sure that someone will let me know.

Accuracy is how well a measurement compares to a standard. If my one
meter measuring stick is not one meter long, every measurement I make
with it will be inaccurate.

Precision is the variation among measurements. Even if the measuring
stick is absolutely one meter long, every time I make a measurement, I
may misplace it a bit. Each realization of the same measurement will be

In most cases, it is better to be imprecise but accurate than inaccurate
but precise. In the former you can be reasonably sure that the
"correct" result is in there somewhere. In the latter you can be
absolutely sure that your measurement is wrong. A great simplification,
but it works with the layman.

Now I will take the leap of applying this to time. (Pun intended.)

UTC provides precise time intervals for most practical purposes.
However, it is inaccurate as the difference between UTC and time scales
based on Earth rotation grows. I know precisely at the end of 86,400 SI
seconds, that my perception of where I am in space is wrong.

This is why we are members of the Precise Time and Time Interval
community -- not the Accurate Time and Time Interval Community, I think.

Dave Finkleman
Senior Scientist
Center for Space Standards and Innovation
Analytical Graphics, Inc.
7150 Campus Drive
Colorado Springs, CO 80920

Phone: 719-510-8282 or 719-321-4780
Fax: 719-573-9079

Discover CSSI data downloads, technical webinars, publications, and
outreach events at www.CenterForSpace.com.


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