GMAT Verbal & Essays

I have been preparing for GMAT from 3-4 months. But I cannot score above 580. Quant-42-45 and verbal-18-21. I just cannot understand critical reasoning. I am pretty well in SC. I take so much time for RC. I solved official guide-13, official guide for verbal. Please help me with the definite strategy to follow with the books to solve and any other sources for the same.

For CR, I'd suggest at working through something like Powerscore CR, or if you really want to raise the bar, something like LSAT Examcrackers CR (although not all these Q-types are also on the GMAT). What you really need to do is 1) refine your strategy for HOW you're approaching CR (your methodical, step-by-step approach utilizing your scratch pad for maximum benefit) and 2) re-familiarize yourself with the most-common logical fallacies.

For the latter, I'd really take a look at causation/correlation. In statistics, let's say we have data from two variables: x and y. They have a direct relationship. As one increases, the other increases. This could lead scientists to draw a conclusion that one variable causes the change in the other variable. But this isn't necessary true! In fact, some third variable may be affecting both x and y! In order to prove causation and validate that one variable is indeed directly influencing the other and is the reason behind the detected correlation, further studies would need to be conducted, altering parameters, and recording outcomes.
The "Secret": Just because X and Y are connected, doesn't mean X causes Y. Let's take a look at a GMAT question:

A recent study from USC determined that although only 22 percent of students wore USC Trojans paraphernalia to school on a regular basis, over 65 percent of students who passed their finals were wearing USC Trojans clothing at the time. The study concluded that students who wear USC Trojans clothing are more likely to do well on their finals than students who don't.

Which of the following would best strengthen the study's conclusion?

(A) Of the students who failed their finals, none of them have ever been seen in class wearing USC-branded clothing.
(B) The week of finals every year is always on the same week as a big football game.
(C) USC clothing is well-insulated and the classrooms at USC are kept highly air-conditioned.
(D) Many students who did not pass their finals regularly attend tailgate parties.
(E) Students at USC tend to show more school spirit than students at other colleges.

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(A) is the correct response. This is a classic "causation/correlation" argument.
Here our "X" is USC clothing and "Y" is better finals scores. The weakness in the argument is that there is no substantial evidence linking students who wear the clothing to better finals scores. None of the answer choices really give us "substantial" evidence to PROVE a causal relationship, but this piece of evidence would support the study's conclusion if it were true.

Good luck!