an effective comparative writing always contains following elements :
Clarity : To make a sentence effective , author's vue should be seen in clarity . or else there is no meaning of that sentence.
Correctness : the statement should be correct enough to make readers agree with points to make it effective .
Well organised plain for body of sentence : it is very important for prepare the sentence keeping all the valuable points layout in brain .
Arguments and similarities : While the author is comparing things , it should allways contain both arguments and similarities , to make the reader believe and agree with one's point .