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$stdDevSamp (expression)

Definition

$stdDevSamp

Returns the sample standard deviation of numeric values in an array.

Note

Disambiguation

This page describes $stdDevSamp when used as an expression, which returns the sample standard deviation of numeric values in an array.

You can also use $stdDevSamp in these other contexts:

$stdDevSamp (accumulator), which calculates the sample standard deviation of values across a group of documents.
$stdDevSamp (window function), which is used in the $setWindowFields stage to calculate the sample standard deviation of values in a window.

Syntax

When used as an expression, $stdDevSamp has one of these syntaxes:

{ $stdDevSamp: <expression> }

In the preceding syntax, the operand is a single expression that evaluates to an array. $stdDevSamp traverses the array and returns the sample standard deviation of the array's numeric values.

{ $stdDevSamp: [ <expression1>, <expression2>, ... ] }

In the preceding syntax, each expression should evaluate to a single numeric value. $stdDevSamp returns the sample standard deviation of those values. Non-numeric values are ignored.

Behavior

Result Type

$stdDevSamp returns the sample standard deviation of the input values as a double.

Non-Numeric Values

$stdDevSamp ignores non-numeric values, null values, and missing fields. If all input values are non-numeric or missing, $stdDevSamp returns null.

Single Value

If there is only one numeric input value, $stdDevSamp returns null.

Example

Create a testScores collection:

db.testScores.insertMany( [
   { studentId: "2345", scores: [ 62, 81, 80, 75, 68, 72, 74, 78, 69, 77 ] },
   { studentId: "2356", scores: [ 60, 83, 79, 72, 65, 71, 68, 74, 64, 76 ] },
   { studentId: "2358", scores: [ 67, 82, 78, 70, 84, 73, 79, 71, 88, 75 ] },
   { studentId: "2367", scores: [ 64, 72, 77, 58, 91, 66, 83, 55, 94, 61 ] },
   { studentId: "2369", scores: [ 60, 53, 72, 85, 63, 88, 57, 79, 66, 74 ] }
] )

The following pipeline uses $stdDevSamp to calculate the sample standard deviation of each student's test scores:

db.testScores.aggregate( [
   {
      $project: {
         _id: 0,
         studentId: 1,
         scoreStdDev: {
            $stdDevSamp: "$scores"
         }
      }
   }
] )

[
  {
    studentId: '2345',
    scoreStdDev: 5.94792213951879
  },
  {
    studentId: '2356',
    scoreStdDev: 7.130529043797833
  },
  {
    studentId: '2358',
    scoreStdDev: 6.700746227100381
  },
  {
    studentId: '2367',
    scoreStdDev: 13.731148693552349
  },
  {
    studentId: '2369',
    scoreStdDev: 11.888836967695557
  }
]

In the preceding pipeline, $project with $stdDevSamp computes the sample standard deviation of each student's scores array and returns it as scoreStdDev.

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$stdDevPop

$strcasecmp