Database

[{"probability":0.00001,"timestamp":ISODate("2012-10-15T21:26:17Z")},{"probability":0.000003,"timestamp":ISODate("2012-10-15T21:26:17Z")},{"probability":0.000025,"timestamp":ISODate("2012-10-15T21:26:17Z")},{"probability":0.000003,"timestamp":ISODate("2012-10-15T21:26:17Z")},{"probability":0.9,"timestamp":ISODate("2012-10-15T21:26:17Z")},{"probability":0.6,"timestamp":ISODate("2012-10-15T21:26:17Z")},{"probability":0.6,"timestamp":ISODate("2012-10-15T21:26:17Z")}]

Query

db.collection.aggregate([{"$match":{"timestamp":{"$gte":ISODate("2012-10-15T21:26:17Z"),"$lte":ISODate("2012-10-15T21:26:17Z")}}},{"$group":{"_id":"$probability","count":{"$sum":1}}},{"$group":{"_id":null,"less":{"$push":{"$cond":{"if":{"$lte":["$_id",0.001]},"then":{"probability":"$_id","count":"$count"},"else":"$$REMOVE"}}},"greater":{"$push":{"$cond":{"if":{"$gte":["$_id",0.001]},"then":{"probability":"$_id","count":"$count"},"else":"$$REMOVE"}}}}},{"$project":{"_id":0}}])

Result