Standard deviation for Array lengths

[d4nirod]

Given the example output below (edited for brevity) for a specific field of interest from a collection analysis: Tasks, which can optionally have Assignees (array).
Regarding the number of assignees (array length) it would be very useful to have the standard deviation besides the already provided average_length

   "_id" : ObjectId("5a8d71276397ce1a2dd42bbe"), 
   "name" : "assignees", 
   "path" : "assignees", 
   "count" : NumberInt(44), 
   "types" : [
       {
           "name" : "Undefined", 
           "type" : "Undefined", 
           "path" : "assignees", 
           "count" : NumberInt(56), 
           "total_count" : NumberInt(0), 
           "probability" : 0.56, 
           "unique" : NumberInt(1), 
           "has_duplicates" : true
       }, 
       {
           "name" : "Array", 
           "bsonType" : "Array", 
           "path" : "assignees", 
           "count" : NumberInt(30), 
           "types" : [
               {
                   "name" : "DBRef", 
                   "bsonType" : "DBRef", 
                   "path" : "assignees", 
                   "count" : NumberInt(37), 
                   "values" : [
                       DBRef("cw_user", ObjectId("577e7f1f300488c6676b3406")), 
                       DBRef("cw_user", ObjectId("577e7f08300488c6676b33f5")), 
                       DBRef("cw_role", ObjectId("582493383004c0551c10bc5d")), 
                       DBRef("cw_user", ObjectId("577e7f08300488c6676b33f5")), 
                       DBRef("cw_user", ObjectId("577e7f08300488c6676b33f5")), 
                       DBRef("cw_user", ObjectId("577e7f08300488c6676b33f5")), 
                       DBRef("cw_role", ObjectId("5a7c46bd39c3cc64d3683a18")), 
                       DBRef("cw_user", ObjectId("577e7f08300488c6676b33f5")), 
                       DBRef("cw_user", ObjectId("577e7f08300488c6676b33f5")), 
                       DBRef("cw_role", ObjectId("5a7c46bd39c3cc64d3683a18")), 
                       DBRef("cw_user", ObjectId("577e7f85300488c6676b344c")), 
                       DBRef("cw_user", ObjectId("577e7f08300488c6676b33f5")), 
                       DBRef("cw_user", ObjectId("5a8d51cd6397ce5a3b44496c"))
                   ], 
                   "total_count" : NumberInt(0), 
                   "probability" : NumberInt(1), 
                   "unique" : NumberInt(1), 
                   "has_duplicates" : true
               }
           ], 
           "lengths" : [
               NumberInt(2), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(2), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(3), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(3), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(1), 
               NumberInt(2), 
               NumberInt(1), 
               NumberInt(0), 
               NumberInt(2), 
               NumberInt(2), 
               NumberInt(0), 
               NumberInt(1)
           ], 
           "total_count" : NumberInt(37), 
           "probability" : 0.3, 
           "average_length" : 1.2333333333333334
       }, 
       {
           "name" : "Null", 
           "bsonType" : "Null", 
           "path" : "assignees", 
           "count" : NumberInt(14), 
           "total_count" : NumberInt(0), 
           "probability" : 0.14, 
           "unique" : NumberInt(1), 
           "has_duplicates" : true
       }
   ], 
   "total_count" : NumberInt(100), 
   "type" : [
       "Undefined", 
       "Array", 
       "Null"
   ], 
   "has_duplicates" : true, 
   "probability" : 0.44
}

[pzrq]

Which one? Or both?
https://math.stackexchange.com/questions/15098/sample-standard-deviation-vs-population-standard-deviation

[d4nirod]

• From my limited understanding they're two different things if you are dealing with the whole dataset (population) or a subset (sample) but want to extrapolate the results of the latter to the whole.
• Mongo aggregations easily give you both with $stdDevPop and $stdDevSamp

So why not have both either by default or with some option?