# standard deviation (`stddev`

)

The standard deviation is a measure that is used to quantify the amount of variation or dispersion of a set of data values.

A low standard deviation indicates that the data points tend to be close to the mean (also called the expected value) of the set, while a high standard deviation indicates that the data points are spread out over a wider range of values.

## how to use

Use it in alerts like this:

` alarm: my_alert`

on: my_chart

lookup: stddev -1m unaligned of my_dimension

warn: $this > 1000

`stdev`

does not change the units. For example, if the chart units is `requests/sec`

, the standard
deviation will be again expressed in the same units.

It can also be used in APIs and badges as `&group=stddev`

in the URL.

## Examples

Examining last 1 minute `successful`

web server responses:

## References

Check https://en.wikipedia.org/wiki/Standard_deviation.

# Coefficient of variation (`cv`

)

This query is also available as

`rsd`

.

The coefficient of variation (`cv`

), also known as relative standard deviation (`rsd`

),
is a standardized measure of dispersion of a probability distribution or frequency distribution.

It is defined as the ratio of the **standard deviation** to the **mean**.

In simple terms, it gives the percentage of change. So, if the average value of a metric is 1000
and its standard deviation is 100 (meaning that it variates from 900 to 1100), then `cv`

is 10%.

This is an easy way to check the % variation, without using absolute values.

For example, you may trigger an alert if your web server requests/sec `cv`

is above 20 (`%`

)
over the last minute. So if your web server was serving 1000 reqs/sec over the last minute,
it will trigger the alert if had spikes below 800/sec or above 1200/sec.

## how to use

Use it in alerts like this:

` alarm: my_alert`

on: my_chart

lookup: cv -1m unaligned of my_dimension

units: %

warn: $this > 20

The units reported by `cv`

is always `%`

.

It can also be used in APIs and badges as `&group=cv`

in the URL.

## Examples

Examining last 1 minute `successful`

web server responses:

## References

Check https://en.wikipedia.org/wiki/Coefficient_of_variation.

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