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# Standard Deviation (StdDev)

## Description

In probability theory and statistics, **standard deviation** is a measure of the variability or dispersion of a population, a data set, or a probability distribution. A low **standard deviation** indicates that the data points tend to be very close to the same value (the mean), while high **standard deviation** indicates that the data are “spread out” over a large range of values.

... Courtesy of [Wikipedia](https://en.wikipedia.org/wiki/Standard_deviation)

## Syntax

`StdDev(int period)`

`StdDev(ISeries\<double\> input, int period)`

**Returns default value**

`StdDev(int period)[int barsAgo]`

`StdDev(ISeries\<double\> input, int period)[int barsAgo]`

## Return Value

**double;** Accessing this method via an index value `[int barsAgo]` returns the indicator value of the referenced bar.

## Parameters

| Parameter | Description                                                                                     |
| --------- | ----------------------------------------------------------------------------------------------- |
| input     | Indicator source data ([Series\<T>](/developer/desktop-sdk/references/common/iseriest/seriest)) |
| period    | Number of bars used in the calculation                                                          |

## Examples

```csharp
// Prints the current value of a 20 period **StdDev** using default price type
double value = StdDev(20)[0];
Print("The current **StdDev** value is " + value.ToString());

// Prints the current value of a 20 period **StdDev** using high price type
double value = StdDev(High, 20)[0];
Print("The current **StdDev** value is " + value.ToString());
```

## Source Code

You can view this indicator method source code by selecting the menu New > **NinjaScript** Editor > Indicators within the **NinjaTrader** Control Center window.