Because I thought that criteria should be accepted , I decided to check Webster's Dictionary. According to Webster's Dictionary, Criterion is a singular word and criteria is a plural word. However it also states that criteria has been used as a singular word for about 50 years and is increasingly accepted in both speech and writing.
i don't understand why the article is used in some cases but not in others. In this case, "a", I assume, is the article for qualidade, but is it really necessary grammatically in Portuguese (in English, of course, it is not unless one is emphasizing THE quality (which would also imply "of our particular product or service")