Measure the "quality" of a description

I have interviewed 36 people, divided in three groups of 12 people. For one group I asked to describe a song they just heard. The second group had to describe a song by observing only a graphical representation of the song and the last group had the song and the representation.

My intention was to measure how the graphical representation helped the users to increase the quality of their description.

Now, the problem is, are there works with methodologies similar to mine? How do you think I could measure the quality of a description? Number of words? Number of adjectives used? With an entropy calculator? I committed the mistake of not consulting first a methodology to inspire my process.