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$stdDevSamp

The `$stdDevSamp` operator calculates the standard deviation by taking a specified sample of the values of a field. The standard deviation is calculated by taking a random sample of the specified size. If a precise standard deviation is needed, $stdDevPop must be used instead.

Syntax

{
  $stdDevSamp: <fieldName>
}

Parameters

fieldNamestringrequired

The field whose values are used to calculate the standard deviation of the specified sample size.

Examples

Sample Data

{
  "_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
  "name": "First Up Consultants | Beverage Shop",
  "sales": {
    "totalSales": 75670,
    "salesByCategory": [
      { "categoryName": "Wine Accessories", "totalSales": 34440 }
    ]
  }
}

Calculate the standard deviation of total sales

Calculate the standard deviation of total sales across stores by taking a random sample.

This query takes a sample of 10 documents matching the filtering criteria and calculates standard deviation.

Query:

db.stores.aggregate([
  {
    $match: { company: "Fourth Coffee" }
  },
  {
    $sample: { size: 10 }
  },
  {
    $group: {
      _id: "$company",
      stdDev: {
        $stdDevSamp: "$sales.totalSales"
      }
    }
  }
])

Output:

[
  {
    "_id": "Fourth Coffee",
    "stdDev": 22040.044055209048
  }
]

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