# stddev\_pop

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Returns the population standard deviation of non-NULL values.

## Syntax

```scopeql
stddev_pop(<expr>)
```

## Arguments

| Argument | Type                      | Description  |
| -------- | ------------------------- | ------------ |
| `<expr>` | `int`, `uint`, or `float` | Input value. |

## Returns

Returns `float`. Ignores NULL values. Returns NULL for no non-NULL values and `0.0` for one non-NULL value.

## Examples

Compare request latency across channels, treating these rows as all requests in the reporting period:

```scopeql
VALUES
    ('web', 100.0), ('web', 120.0), ('web', 140.0),
    ('mobile', 60.0), ('mobile', 120.0), ('mobile', 180.0)
SELECT $0 AS channel, $1 AS latency_ms
GROUP BY channel
AGGREGATE avg(latency_ms) AS avg_ms, stddev_pop(latency_ms) AS stddev_ms
SELECT channel, avg_ms, round(stddev_ms * 100.0) / 100.0 AS stddev_ms
ORDER BY channel
```

```text
+---------+--------+-----------+
| channel | avg_ms | stddev_ms |
+---------+--------+-----------+
| mobile  | 120.0  | 48.99     |
| web     | 120.0  | 16.33     |
+---------+--------+-----------+
```

Both channels have the same mean, but mobile durations are more spread out. The population standard deviation is measured in ms; results are rounded to two decimal places.