Standard set of terms for precision, accuracy, related concepts.
Gary E. Miller
gem at rellim.com
Mon Apr 24 17:07:47 UTC 2017
On Mon, 24 Apr 2017 18:49:39 +0200
Kurt Roeckx <kurt at roeckx.be> wrote:
> I think one of the problems is that such terms are used at many
> different places. And you might need to be more specific at which
> one you mean. That is, there might be variables called precision,
> but not all of them mean exactly the same. A text that works for
> one might be wrong for the other.
Yup. 'precision' is used many places in NTP for several different
and incompatible concepts.
And 'accuracy' is a useless and overbroad term for NTP. NTP talks
instead of 'offset' and 'jitter' computed between what ntpd thinks the
time is and each of it's peers, serversm refclocks and local system
> So I suggest you start by finding out where you all use such
And that taks has been ongoing. Ian has spent a lot of time just
documenting the units for many variables. Documenting what the actually
do will be a very large job.
So for my submission, I'll stand by the glossary at the bottom of
the ntpviz html output. Included below for reference, but the html formatting is lost. No need to create yet another document.
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem at rellim.com Tel:+1 541 382 8588
Veritas liberabit vos. -- Quid est veritas?
"If you can’t measure it, you can’t improve it." - Lord Kelvin
The difference between the ntpd calculated frequency and the local system clock frequency (usually in parts per million, ppm)
The short term change in a value
One thousandth of a second = 0.001s
The arithmetic mean: the sum of all the values divided by the number of values.
One billionth of a second, also one thousandth of a microsecond, 0.000000001s.
The value below which a given percentage of values fall.
ppb, parts per billion:
Ratio between two values. These following are all the same: 1 ppb, one in one billion, 1/1,000,000,000, 0.000,000,001, and 0.000,000,1%
ppm, parts per million:
Ratio between two values. These following are all the same: 1 ppm, one in one million, 1/1,000,000, 0.000,001, and 0.000,1%
ppt, parts per thousand:
Ratio between two values. These following are all the same: 1 ppt, one in one thousand, 1/1,000, 0.001, and 0.1%
Reference clock, a local GPS module or other local source of time.
Any clock reached over the network, LAN or WAN. Also called a peer or server.
The difference between the ntpd calculated time and the local system clock's time. Also called phase offset.
Any remote clock or reference clock used as a source of time.
Sigma denotes the standard deviation (SD) and is centered on the arithmetic mean of the data set. The SD is simply the square root of the variance of the data set. Two sigma is simply twice the standard deviation. Three sigma is three times sigma. Smaller is better.
µs, us, microsecond:
One millionth of a second, also one thousandth of a millisecond, 0.000,001s.
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