DS answer choice mistakes

[pagani88]

Hello community,

I have encountered in a serious trap multiple times. When doing OG2017 DS section (almost finished btw), I have made quite many mistakes when trying to choose between answer choices C and E. I succeed to identify that A and B are insufficient separately, but then I choose wrong answers, I don't know why.
Most of the time I chose E, but it turned out to be C. Choosing C, seldom turns out to be E, however. But it is almost equal throughput. 9 E to C and 6 C to E mistakes.

Our dear experts, does anyone have any advice how to tackle such a hurdle?

Thank you for your help in advance,
Artur

[ceilidh.erickson]

Without seeing your work, it's impossible to say for sure what your mistake is, but it's likely that you don't yet have a solid DS process.

Here is a very comprehensive list of resources for DS strategy and common DS mistakes: https://www.beatthegmat.com/data-suffici ... tml#797668
Start by reading this.

And here are some common mistakes based on the answers you described:

"Answer was C but I chose E"
It's likely that your notation is faulty. You correctly see that stmts 1 & 2 are insufficient on their own, but you don't write down what you *do* learn from them. When you combine the stmts, you're missing key pieces of what they give you. Here's a very simplistic example:

Q: what is the value of x?

(1) x^2 = 16
"Not sufficient because squares have multiple values"

(2) x < 0
"Not sufficient because infinite values"

(1) & (2)
"Both stmts give multiple values, so must not be enough. E."

Here, if the student had written:
(1) x = 4 or -4
(2) x neg

... it would be much easier to see that combining them narrowed it to one possible value. Try writing more down!

"Answer was E but I chose C"
This usually happens when students make assumptions: "they didn't work separately, but I bet they work together." Never assume sufficiency - always test values to try to prove *insufficiency*.

Here's a common example from OG 2016, DS #113:

Is rst = 1?
(1) rs = 1
(2) st = 1

(1) "Not sufficient because we don't know about t."

(2) "Not sufficient because we don't know about r."

(1) & (2)
"We have info about all 3, looks like they're all 1. Sufficient."

Wrong! Try testing r = -1, s = -1, t = -1. Or r = 1/2, s = 2, t = 1/2. The product of all 3 might not be 1, so it's insufficient.

Hope this helps!