Measure the accuracy when a field may contain array data. There are two scenarios
1. includeScalar is set to true (e.g. find({a: {$gt: 10}}))
2. includeScalar is set to false (e.g. find({a: {$elemMatch: {$gt: 10}}}))

Note that a field may contain both array and scalar data. For example:

> db.coll.find();
{a: 1}
{a: [2]}
{a: [3, ,4]}