As a 5 element practitioner, that is a practitioner trained to diagnose and treat using CF, I think this is an interesting question. However, for dissertation purposes, it is muddled. The 'most common' CF, meaning what is the distribution in the public at large; or the most common CF seen in practice? So, I would respectfully suggest you need to clarify this before you go ahead, although I'm sure your supervisor will point this out. I used to be a university lecturer so, forgive my pedantry!
However, the subject is interesting as, indeed, there seems to be a trend that practitioners notice, inasmuch as they report having a predominant CF in patients. Now, whether that means their style attracts a particular CF, or the practitioners CF is interacting with a patient's element or there is a genuine phenomenon, I can't say. Theoretically speaking, Fire should be the CF most frequently found in the population as a whole, as there are twice as many 'opportunities' for the body/mind/spirit to find the element. How you could validate this in a study is quite challenging so, keep me posted!