# $stdDevSamp (aggregation)

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## Definition

`$stdDevSamp`

*New in version 3.2*.Calculates the sample standard deviation of the input values. Use if the values encompass a sample of a population of data from which to generalize about the population.

`$stdDevSamp`

ignores non-numeric values.If the values represent the entire population of data or you do not wish to generalize about a larger population, use

`$stdDevPop`

instead.`$stdDevSamp`

is available in the in the following stages:`$group`

`$project`

`$addFields`

(Available starting in MongoDB 3.4)`$set`

(Available starting in MongoDB 4.2)`$replaceRoot`

(Available starting in MongoDB 3.4)`$replaceWith`

(Available starting in MongoDB 4.2)`$match`

stage that includes an`$expr`

expression

When used in the

`$group`

stage,`$stdDevSamp`

has the following syntax and returns the sample standard deviation of the specified expression for a group of documents that share the same group by key:{ $stdDevSamp: <expression> } When used in the other supported stages,

`$stdDevSamp`

returns the sample standard deviation of the specified expression or list of expressions for each document and has one of two syntaxes:`$stdDevSamp`

has one specified expression as its operand:{ $stdDevSamp: <expression> } `$stdDevSamp`

has a list of specified expressions as its operand:{ $stdDevSamp: [ <expression1>, <expression2> ... ] }

The argument for

`$stdDevSamp`

can be any expression as long as it resolves to an array. For more information on expressions, see Expressions.

## Behavior

### Non-numeric Values

`$stdDevSamp`

ignores non-numeric values. If all operands for a
sum are non-numeric, `$stdDevSamp`

returns `null`

.

### Single Value

If the sample consists of a single numeric value, `$stdDevSamp`

returns `null`

.

### Array Operand

In the `$group`

stage, if the expression resolves to an
array, `$stdDevSamp`

treats the operand as a non-numerical value.

In the other supported stages:

- With a single expression as its operand, if the expression resolves
to an array,
`$stdDevSamp`

traverses into the array to operate on the numerical elements of the array to return a single value. - With a list of expressions as its operand, if any of the expressions
resolves to an array,
`$stdDevSamp`

does**not**traverse into the array but instead treats the array as a non-numerical value.

## Example

A collection `users`

contains documents with the following fields:

{_id: 0, username: "user0", age: 20} {_id: 1, username: "user1", age: 42} {_id: 2, username: "user2", age: 28} ...

To calculate the standard deviation of a sample of users, following
aggregation operation first uses the `$sample`

pipeline to
sample 100 users, and then uses `$stdDevSamp`

calculates the
standard deviation for the sampled users.

db.users.aggregate( [ { $sample: { size: 100 } }, { $group: { _id: null, ageStdDev: { $stdDevSamp: "$age" } } } ] )

The operation returns a result like the following:

{ "_id" : null, "ageStdDev" : 7.811258386185771 }