# 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 alarms like this:

alarm: my_alarm
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:

# 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 alarm 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 alarm if had spikes below 800/sec or above 1200/sec.

## how to use​

Use it in alarms like this:

alarm: my_alarm
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​

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