$stdDevSamp (accumulator operator)

Result Type

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

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 and $setWindowFields stages, 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 numeric 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-numeric value.

Window Values

Behavior with values in a $setWindowFields stage window:

• Ignores non-numeric values, null values, and missing fields in a window.
• If the window is empty, returns null.
• If the window contains a NaN value, returns null.
• If the window contains Infinity values, returns null.
• If none of the previous points apply, returns a double value.

Use in $group Stage

A collection users contains documents with the following fields:

db.users.insertMany( [
   { _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 }

Use in $setWindowFields Stage

Create a cakeSales collection that contains cake sales in the states of California (CA) and Washington (WA):

db.cakeSales.insertMany( [
   { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
     state: "CA", price: 13, quantity: 120 },
   { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
     state: "WA", price: 14, quantity: 140 },
   { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
     state: "CA", price: 12, quantity: 145 },
   { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
     state: "WA", price: 13, quantity: 104 },
   { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
     state: "CA", price: 41, quantity: 162 },
   { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
     state: "WA", price: 43, quantity: 134 }
] )

This example uses $stdDevSamp in the $setWindowFields stage to output the sample standard deviation of the quantity values of the cake sales for each state:

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            stdDevSampQuantityForState: {
               $stdDevSamp: "$quantity",
               window: {
                  documents: [ "unbounded", "current" ]
               }
            }
         }
      }
   }
] )

In the example:

• partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.
• sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.

• output sets the stdDevSampQuantityForState field to the sample standard deviation of the quantity values using $stdDevSamp that is run in a documents window.

  The window contains documents between an unbounded lower limit and the current document in the output. This means $stdDevSamp returns the sample standard deviation of the quantity values for the documents between the beginning of the partition and the current document.

In this output, the sample standard deviation quantity value for CA and WA is shown in the stdDevSampQuantityForState field:

{ _id: 4, type: "strawberry", orderDate: ISODate("2019-05-18T16:09:01Z"),
  state: "CA", price: 41, quantity: 162, stdDevSampQuantityForState: null }
{ _id: 0, type: "chocolate", orderDate: ISODate("2020-05-18T14:10:30Z"),
  state: "CA", price: 13, quantity: 120, stdDevSampQuantityForState: 29.698484809834994 }
{ _id: 2, type: "vanilla", orderDate: ISODate("2021-01-11T06:31:15Z"),
  state: "CA", price: 12, quantity: 145, stdDevSampQuantityForState: 21.1266025033211 }
{ _id: 5, type: "strawberry", orderDate: ISODate("2019-01-08T06:12:03Z"),
  state: "WA", price: 43, quantity: 134, stdDevSampQuantityForState: null }
{ _id: 3, type: "vanilla", orderDate: ISODate("2020-02-08T13:13:23Z"),
  state: "WA", price: 13, quantity: 104, stdDevSampQuantityForState: 21.213203435596427 }
{ _id: 1, type: "chocolate", orderDate: ISODate("2021-03-20T11:30:05Z"),
  state: "WA", price: 14, quantity: 140, stdDevSampQuantityForState: 19.28730152198591 }