# Bollinger Bands

`bb`

A moving average with bands set by the standard deviation.

## Inputs

| Name | Type | Default | Range |
| --- | --- | --- | --- |
| source | source | close |  |
| period | int | 20 | 1 to 5000 |
| deviations | float | 2 | 0 to 10 |

## Outputs

| Name | Kind |
| --- | --- |
| upper | value |
| middle | value |
| lower | value |

Warm-up: period - 1 bars (19 at the defaults).

Repaints: No.

## Formula

middle = the simple average of the source over period

upper = middle + deviations * the population standard deviation of the same window, lower = middle - deviations * the same