# 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 . --- # 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 .