Standard set of terms for precision, accuracy, related concepts.

[Achim Gratz]

Gary E. Miller writes:
> Glossary:
>
> frequency offset:
>     The difference between the ntpd calculated frequency and the local system clock frequency (usually in parts per million, ppm)

It's actually the current correction ntpd applies to the system clock
(for ppm, the correct unit would be µs/s).  It would converge to your
definition for a stationary system if all noise sources were unbiased.

> jitter, dispersion:
>     The short term change in a value

That's too short for an explanation.  Jitter is the deviation from an
idealized periodical signal and can refer to both the devation at some
discrete time (clock event) or more commonly the resulting
distribution.  The dispersion is the width of that distribution and can
be expressed in different ways depending on the type of distribution.
> 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%

When talking about precise frequency measurements I would rather expect
ppt to refer to "parts per trillion".

"Parts per thousand" is usually expressed as "per mille [‰]"

> σ, sigma:
>     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.

Sigma is the measure of dispersion for normal distributions.  It isn't
centered on anything and the rest of that explanation is either
misleading or redundant.


Regards,
Achim.
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