$stdDevSamp (Sample StdDev Accumulator)

Last updated: 2026-10-09

$stdDevSamp (Sample StdDev Accumulator)

$stdDevSamp is an accumulator computing the sample standard deviation of a field across a group, dividing by N-1. Use it when data is a sample of a larger population — the common case. Non-numeric values are ignored; the denominator differs from $stdDevPop.

📝 Syntax

{ $group: { _id: <key>, dev: { $stdDevSamp: "$field" } } }

Returns：Sample standard deviation (non-numeric values ignored)

▶ Example

Run the commands below in your local mongosh (requires a running mongod):

// $stdDevSamp: sample stddev of scores per class
db.scores.aggregate([
  { $group: { _id: '$class', dev: { $stdDevSamp: '$score' } } }
])

Output：

{ "_id": "A", "dev": 13.02 }

📌 Notes

• Sample stddev divides by N-1
• Null and non-numeric values are ignored
• Needs at least two numeric values per group, else null

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