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## Re: Calculating Jitter

• From: Dragos Ruiu
• Date: Wed Jun 15 04:26:06 2005

```On June 10, 2005 10:07 am, Eric Frazier wrote:
> At 09:56 AM 6/10/2005, Fred Baker wrote:
> >you saw marshall's comment. If you're interested in a moving average, he's
> >pretty close.
> >
> >If I understood your question, though, you simply wanted to quantify the
> >jitter in a set of samples. I should think there are two obvious
> >definitions there.
> >
> >A statistician would look, I should think, at the variance of the set.
> >Reaching for my CRC book of standard math formulae and tables, it defines
> >the variance as the square of the standard deviation of the set, which is
> >to say
>
> That is one thing I have never understood, if you can pretty much just look
> at a standard dev and see it is high, and yeah that means your numbers are
> flopping all over the place, then what good is the square of it? Does it
> just make graphing better in some way?

Pedantic mode on.

Jitter != Variance

Variance is how spread out a set is (hence the squaring). [it has further
subclasses at sample variance and population variance]

To try to explain it a little more simply than CHI-square distributions
and formulas....
(Mark Twain said, why use a 25 cent word when a nickel one will do... :-)

The squaring lets you get an indication of the range with added bias
to wildly differing samples so they won't be drowned out by the large
number of samples close to the mean.

Sample variance is the sum of the squares of the differences from the mean
divided by number of samples. It is a numeric value.

But...

Jitter is the measure of deviation from a predetermined constant clock rate.
It can also be the phase deviation of a signal.

You can think of it as clock skew or drift or the uncertainty level in timing.
Jitter is normally expressed as a plus minus range in either peak, mean, or
rms.

They are similar but not the same thing.

Pedantic mode off...

cheers,
--dr

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