Dear Adriana,

I found a paper  on the "ervice with a smile". In your case it is not about a smile, but you can look for how the authors coded and evluated the data. I hope it helps.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.608.6453&rep=rep1&type=pdf

or the same paper here:

https://www.jstor.org/stable/20159829?seq=1#page_scan_tab_contents

Sincerely,

Eugene