Data Sufficiency

I came across this post in some forum last year. I hope this advice is as helpful to you as it was to me:

DS is definitely an acquired taste... It's really important, though. On my exam I got about 50% DS. Kaplan claims it's about 1/3, so maybe quite a few of mine were experimental - still, even 1/3 represents a large chunk of your quant score!

I had trouble with DS at first, too, but got better at it with practice. I don't know where you are in your prep, but the Kaplan books (both the general GMAT book and the Math workbook) have some specific DS strategies:
1. Focus on the question stem.
2. Look at each statement separately.
3. Look at both statements in combination.

I find that the most important thing is to keep all the pieces separate - what's given in the stem vs. each of the two statements. Most careless errors on DS come when you subconsciously carry over some info from statement 1 to statement 2 (i.e. you're not actually looking at statement 2 on its own).

Obviously, quite apart from the unusual question format, you also need to know the content on which the question is based. That's where it helps to really know your number properties, divisibility, exponents, inequalities etc.

Also, on the more difficult DS questions, there is often some type of trap for the unwary. For example, most people know that if you can set up two linear equations with two unknowns, you can solve for both variables. So the natural instinct when each of the statements can be translated into an equation is to assume that the answer must be C. However, sometimes the two equations may not be independent of each other (e.g. x + 2y = 3 and 2x + 4y = 6, which are just two versions of the same equation).

At other times, the question stem may actually ask for a sum (e.g. a+b), and even though there are two variables, a single equation (e.g. 4a + 4b = 20) may be sufficient, since you're only asked to find the sum, not the individual values of a and b.

Finally, another common DS trap occurs with Yes/No questions, for example "Is x divisible by 2?". The trick here is that in DS, you are supposed to determine whether the question can be definitively answered based on the given info. So if statement 1 says "X is odd", we can in fact answer the question (in this case, the answer is "no"). So in a Yes/No question, it doesn't matter whether the answer turns out to be Yes or No - all that matters is whether the given info is sufficient to allow you to find the answer.

The best way to improve DS is really just to understand the basic mechanics, read the question and statements VERY carefully, beware of the traps, and then practice, practice, practice. I went through all the DS questions in OG, then again through all the ones I found problematic, and finally a third round on the ones I STILL had problems with. In my debriefing post there is a link where you can download the OG practice grid I used. It really helps if you can figure out what specific aspects are giving you problems and what types of errors you tend to make - that's how I became aware of the things I mentioned above.