GMAT Data Sufficiency Strategy - Don’t Contradict Yourself

True to their name, Data Sufficiency questions ask you to determine when you will have enough information to make a conclusive decision.  In doing so, these questions can assess your ability to plan ahead for a task; to elicit an effective return-on-investment (remember, you can’t use both statements if one of them is, alone, sufficient), to find flaws with conventional wisdom, and to think flexibly. Data Sufficiency questions also strike fear and loathing in the hearts of many GMAT examinees, but hold a special place in the hearts of a select few who love the nuance that these questions permit.  

There’s a hard-and-fast rule regarding Data Sufficiency that people don’t know and use as much as they should: the statements can never contradict each other.  Knowing this, if your answers for statement 1 and statement 2 are different, you must go back and reconsider your math; as Boston GMAT tutor David says, that’s an “answer choice F”, meaning that you just effed up the math somehow.

Consider the question:

Is x < 0?

1) x^2 = 9x

2) The absolute value of x = -x

Did you get x = 9 for statement 1, meaning a definitive NO? 
And x is negative for statement 2 for a definitive YES?  Most do.  But how can x be both “negative” and “9? at the same time?  Clearly, as these statements contradict each other, we messed up the math somehow. 

What did we  forget? 
In both cases, x could be 0.  In statement 1 that still gives us “NO” but in statement 2 that gives us an insufficient.  So the answer is A, not D as many might think.  And it was recognizing this inconsistency with both statements – remember, statements are facts…they must be true! – allowed us to catch that potential mistake.