According to Chao (1965）：
School grammars usually prescribe what is correct and what is incorrect, what is grammatical and what is ungrammatical. A descriptive grammar, on the other hand, is concerned with stating the facts of the language without passing any value judgment on them. However, the difference is rather one of form of statement and relative emphasis than one of content. For example, to say “Don’t say ‘like I do’, but say ‘as I do’” is a prescriptive statement, while “people of certain educational or economic class say one and people of another class say the other” is a descriptive statement. One is translatable into the other. The prescriptive statement is a categorical imperative, while the other, if stated in the form “if you want to be classed as a member of a certain class, you must say such and such” is a hypothetical imperative. Moreover, the statement that Keats said： “They raven down scenery like children do sweetmeat's”is both a historical and a descriptive statement. There is usually no issue in the arguments between descriptivists and prescriptivists if they explicate what they are talking about.
Chao Y R. A grammar of spoken Chinese[M]. Univ of California Press, 1965.