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# `stddev`

## Description

Returns the standard deviation of a numerical expression within a group.

* Useful for measuring how much values vary around the mean.
* Best applied when a complete dataset is available (for sampled data, use `sample_stddev`).

## Syntax

```
stddev(expression: number): number
```

## Arguments

| Name         | Type     | Required | Description                                               |
| ------------ | -------- | -------- | --------------------------------------------------------- |
| `expression` | `number` | **true** | Numerical expression whose standard deviation is computed |

## Example

**Use case: Calculate the standard deviation of API latency**

Traces contain a `duration` field representing execution time. Computing its standard deviation helps you understand the variability of API performance.

### Example data

```
{ "operationName": "MalfunctioningDatabase", "duration": 10100 },

{ "operationName": "checkout", "duration": 350000 },

{ "operationName": "brokenPrinter", "duration": 20000 },

{ "operationName": "ScheduledEvent", "duration": 1500000 },

{ "operationName": "checkout", "duration": 100000 },

{ "operationName": "MalfunctioningDatabase", "duration": 10050 },

{ "operationName": "brokenPrinter", "duration": 400000 },

{ "operationName": "ScheduledEvent", "duration": 900000 },

{ "operationName": "checkout", "duration": 600000 },

{ "operationName": "brokenPrinter", "duration": 800000 },

{ "operationName": "ScheduledEvent", "duration": 1200000 },

{ "operationName": "checkout", "duration": 500000 },

{ "operationName": "MalfunctioningDatabase", "duration": 9950 },

{ "operationName": "ScheduledEvent", "duration": 500000 },

{ "operationName": "brokenPrinter", "duration": 1000000 },

{ "operationName": "checkout", "duration": 250000 },

{ "operationName": "MalfunctioningDatabase", "duration": 10000 },

{ "operationName": "brokenPrinter", "duration": 600000 },

{ "operationName": "ScheduledEvent", "duration": 50000 },

{ "operationName": "MalfunctioningDatabase", "duration": 9800 }
```

### Example query

```
groupby $m.operationName aggregate stddev($m.duration) as latency_stddev
```

### Example output

| operationName          | latency\_stddev    |
| ---------------------- | ------------------ |
| checkout               | 241610.80691335752 |
| brokenPrinter          | 279989.950525226   |
| ScheduledEvent         | 343380.2645421216  |
| MalfunctioningDatabase | 554.8304396318806  |