This picture is taken from here.
In that video is mentioned that contra-rotating propeller increases efficiency up to 6-16%. Regardless of the number that could be vary from one airplane to another, what I am asking here are: how the efficiency increase is calculated, how it is compared, to what parameter the efficiency increased is compared? Is it representing overall efficiency after calculated including the weight it added? The noise it created we may neglect, not part of my question.