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# 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:
- 
- 
- 
- 
## References
Check [https://en.wikipedia.org/wiki/Standard_deviation](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 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
Check [https://en.wikipedia.org/wiki/Coefficient_of_variation](https://en.wikipedia.org/wiki/Coefficient_of_variation).
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