Mongo average aggregation query with no group

So if your data actually was numeric which is it not and your intention is to exclude the documents that have a "greater than zero" value then you include a $match statement in your aggregation pipeline in order to "filter" out these documents:

db.EvaluatedSentiments.aggregate([
    { "$match": {
        "bvc": { "$gt": 0 }
    }},
    { "$group": {
        "_id": null,
        "bvc": { "$avg": "$bvc" }
    }}
])

First of all store numerical values as numbers. Afterwards you can use a simple statement to calculate the average:

db.collection.aggregate([{ 
  "$group": {
    "_id": null, 
    "avg_bvc": { "$avg": "$bvc" } 
  } 
}])

You can simply use more $avg aggregation operators to get averages for your other numeric fields:

db.collection.aggregate([{ 
  "$group": {
    "_id": null, 
    "avg_bvc": { "$avg": "$bvc" }, 
    "avg_dollar": { "$avg": "$dollar" } 
  } 
}])