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

Gary E. Miller gem at
Mon Apr 24 19:40:21 UTC 2017

Yo Kurt!

On Mon, 24 Apr 2017 19:30:03 +0200
Kurt Roeckx <kurt at> wrote:

> On Mon, Apr 24, 2017 at 10:07:47AM -0700, Gary E. Miller wrote:
> > frequency offset:
> >     The difference between the ntpd calculated frequency and the
> > local system clock frequency (usually in parts per million, ppm)
> > jitter, dispersion: The short term change in a value  
> For me jitter is about chances in the interval. Some examples which
> I think are relevant are:
> - For a PPS signal, the signal generated by the source is not
>   exactly 1 second each time, but on average it's 1 second.
> - For the same PPS signal, there is a variable delay between
>   the source generating the edge and the PC having seen it
>   and measured the time.
> - A packet send over a network doesn't always have the same delay.

So put that in a nice two sentence explanatory paragraph.  Something
that fits at the bottom of an ntpviz page, suitable for newbies.

> dispersion for me is the standard deviation

Like yes, but standard deviation is a very precise math formula.

    σ = standard deviation

    xi = each value of dataset

    x = the arithmetic mean of the data (This symbol will be indicated
        as mean from now)

    N = the total number of data points

    ∑ (xi - mean)^2 = The sum of (xi - mean)^2 for all datapoints

Leading to:

    σ = √[ ∑(x-mean)^2 / N ]

Very few sqrt() in ntpd, and none near dispersion.  Comceptually they
are similar, measures of the population variance, but σ = standard deviation
is a very specific thing not used in ntpd.

σ is used in ntpviz.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at  Tel:+1 541 382 8588

	    Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin
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