Hi mohitgupta1994,
Questions in the content areas of the
OG are generally arranged in increasing order of difficulty (although you'll find some "easy" questions among the "hard" ones, and vice-versa).
When it comes to training for the GMAT, practice problems and quizzes are great ways to build your skills, learn new content, and discover any weaknesses in your approach. However, the only REAL way to assess your comfort level with the material is to take a FULL LENGTH CAT (with the Essay and IR sections), so that you can see how you perform in the context of the ENTIRE exam. For example, if you're strong at SCs, but get too tired during the Verbal section and make a bunch of silly mistakes, then you still have some things to work on. Since quizzes and book-work don't adapt and don't necessarily follow the "ratio" of content on the Official GMAT, you might also be working in a way that is not realistic, so you should look at these score results more as a means to find (and fix) your weak areas and less as a gauge for "scoring."
GMAT assassins aren't born, they're made,
Rich