When should a percentage be written as a percentage and when should it be written x out of n? People perceive numbers, percentages and values in different ways depending on what it is and how it is presented.

I'm looking for research on when to communicate with an actual percentage and when it is perceived better to say n out of x.

For example in commercials we hear,


  9 out of 10 dentists recommend


We do not hear


  90% of dentists recommend


I'm currently working on some marketing materials and a study has found 4 out of 5 in one instance and 12 out of 19 in the other. I believe 4 out of 5 works better than 80% but Over 65% sounds better than 12 out of 19.

Does anyone have information about the way humans perceive percentages and how to determine the most favorable format?
Two comments. First: the "frequentist" format appears to be easier to understand than the normed percentages, at least if information needs to be chained as in Bayesian reasoning. On the other hand, probably normed percentages are easier to understand if two fractions are to be compared. 
Second, I'd say that you should only use percentages if your sample is large enough. For example, "4 out of 5 respondents" amounts to 80% but the latter figure would probably suggests higher confidence than is warranted, given that it is really only a single respondent away from 3 out of 5 (60%)…