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1 # standard deviation (`stddev`)
2
3 The standard deviation is a measure that is used to quantify the amount of variation or dispersion
4 of a set of data values.
5
6 A low standard deviation indicates that the data points tend to be close to the mean (also called the
7 expected value) of the set, while a high standard deviation indicates that the data points are spread
8 out over a wider range of values.
9
10 ## how to use
11
12 Use it in alerts like this:
13
14 ```
15 alarm: my_alert
16 on: my_chart
17 lookup: stddev -1m unaligned of my_dimension
18 warn: $this > 1000
19 ```
20
21 `stdev` does not change the units. For example, if the chart units is `requests/sec`, the standard
22 deviation will be again expressed in the same units.
23
24 It can also be used in APIs and badges as `&group=stddev` in the URL.
25
26 ## Examples
27
28 Examining last 1 minute `successful` web server responses:
29
30 - ![](https://registry.my-netdata.io/api/v1/badge.svg?chart=web_log_nginx.response_statuses&dimensions=success&group=min&after=-60&label=min)
31 - ![](https://registry.my-netdata.io/api/v1/badge.svg?chart=web_log_nginx.response_statuses&dimensions=success&group=average&after=-60&label=average&value_color=yellow)
32 - ![](https://registry.my-netdata.io/api/v1/badge.svg?chart=web_log_nginx.response_statuses&dimensions=success&group=stddev&after=-60&label=standard+deviation&value_color=orange)
33 - ![](https://registry.my-netdata.io/api/v1/badge.svg?chart=web_log_nginx.response_statuses&dimensions=success&group=max&after=-60&label=max)
34
35 ## References
36
37 Check <https://en.wikipedia.org/wiki/Standard_deviation>.
38
39 ---
40
41 # Coefficient of variation (`cv`)
42
43 > This query is also available as `rsd`.
44
45 The coefficient of variation (`cv`), also known as relative standard deviation (`rsd`),
46 is a standardized measure of dispersion of a probability distribution or frequency distribution.
47
48 It is defined as the ratio of the **standard deviation** to the **mean**.
49
50 In simple terms, it gives the percentage of change. So, if the average value of a metric is 1000
51 and its standard deviation is 100 (meaning that it variates from 900 to 1100), then `cv` is 10%.
52
53 This is an easy way to check the % variation, without using absolute values.
54
55 For example, you may trigger an alert if your web server requests/sec `cv` is above 20 (`%`)
56 over the last minute. So if your web server was serving 1000 reqs/sec over the last minute,
57 it will trigger the alert if had spikes below 800/sec or above 1200/sec.
58
59 ## how to use
60
61 Use it in alerts like this:
62
63 ```
64 alarm: my_alert
65 on: my_chart
66 lookup: cv -1m unaligned of my_dimension
67 units: %
68 warn: $this > 20
69 ```
70
71 The units reported by `cv` is always `%`.
72
73 It can also be used in APIs and badges as `&group=cv` in the URL.
74
75 ## Examples
76
77 Examining last 1 minute `successful` web server responses:
78
79 - ![](https://registry.my-netdata.io/api/v1/badge.svg?chart=web_log_nginx.response_statuses&dimensions=success&group=min&after=-60&label=min)
80 - ![](https://registry.my-netdata.io/api/v1/badge.svg?chart=web_log_nginx.response_statuses&dimensions=success&group=average&after=-60&label=average&value_color=yellow)
81 - ![](https://registry.my-netdata.io/api/v1/badge.svg?chart=web_log_nginx.response_statuses&dimensions=success&group=cv&after=-60&label=coefficient+of+variation&value_color=orange&units=pcent)
82 - ![](https://registry.my-netdata.io/api/v1/badge.svg?chart=web_log_nginx.response_statuses&dimensions=success&group=max&after=-60&label=max)
83
84 ## References
85
86 Check <https://en.wikipedia.org/wiki/Coefficient_of_variation>.
87
88