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$stdDevSamp (accumulator operator)

Definition

$stdDevSamp

Calculates the sample standard deviation of values across a group of documents.

Use $stdDevSamp if the input values represent a sample of a population of data from which you want to generalize about the population. If the values represent the entire population of data, use $stdDevPop instead.

The $stdDevSamp accumulator is available in these stages:

Note

Disambiguation

This page describes $stdDevSamp when used as an accumulator. Accumulators return an aggregated value across a group of input documents.

You can also use $stdDevSamp in these other contexts:

$stdDevSamp (expression), which returns the sample standard deviation of numeric values in an array.
$stdDevSamp (window function), which is used in the $setWindowFields stage to calculate the sample standard deviation of values in a window.

Syntax

{ $stdDevSamp: <expression> }

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 the input consists of a single numeric value, $stdDevSamp returns null.

Example

The examples on this page use data from the sample_mflix dataset. For details on how to load this dataset into your self-managed MongoDB deployment, see Load the Sample Dataset. If you made any modifications to the sample databases, you may need to drop and recreate the databases to run the examples on this page.

This example returns the five directors with the most varied IMDb ratings among directors with at least 10 movies.

db.movies.aggregate( [
   {
      $match: {
         directors: { $exists: true },
         "imdb.rating": { $type: "double" }
      }
   },
   {
      $unwind: "$directors"
   },
   {
      $group: {
         _id: "$directors",
         movieCount: { $sum: 1 },
         stdDevSampRating: { $stdDevSamp: "$imdb.rating" }
      }
   },
   {
      $match: { movieCount: { $gte: 10 } }
   },
   {
      $sort: { stdDevSampRating: -1 }
   },
   {
      $limit: 5
   }
] )

[
  {
    _id: 'Bob Clark',
    movieCount: 12,
    stdDevSampRating: 1.7167622803617066
  },
  {
    _id: 'John McTiernan',
    movieCount: 10,
    stdDevSampRating: 1.517161091571287
  },
  {
    _id: 'James Cameron',
    movieCount: 10,
    stdDevSampRating: 1.4423745853433652
  },
  {
    _id: 'Sean McNamara',
    movieCount: 13,
    stdDevSampRating: 1.4051188470584879
  },
  {
    _id: 'Robert Rodriguez',
    movieCount: 20,
    stdDevSampRating: 1.303143971526675
  }
]

The preceding pipeline:

Uses $match to filter for movies that have a directors field and a numeric imdb.rating.
Uses $unwind to create a separate document for each director credit.
Uses $group to group by director, counts the movies with $sum, and computes stdDevSampRating using $stdDevSamp on imdb.rating.
Uses a second $match to keep only directors with at least 10 movies.
Uses $sort to sort by stdDevSampRating descending.
Uses $limit to return the top 5 results.

Learn More

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$stdDevPop

$sum