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You may have already heard that GMATPrep 2.0® has been released. I’m pretty excited that we finally have a Mac-compatible program – after all, what better way to spend a random Friday night when I’m bored? Seriously, though, I just tried my first CAT yesterday and I’ve got an IR question that I want to share with you.

Note: IR is not adaptive; in other words, we’re all going to see the same questions when we take the two practice CATs. So, if you haven’t taken your first GMATPrep 2.0® CAT yet, you might just want to bookmark this article right now and wait. Once you’ve taken the test, you can come back and read the rest of the article.

One more note: We have an average of 2.5 minutes per question for IR. GMATPrep doesn’t report the time we spent on each individual question, but I took just about 2 minutes for this one. (Note: I strongly recommend that you keep track of your per-question timing yourself while you take the test. Find a way to do so easily so that it doesn’t distract you while you’re taking the test.)

Try the problem

All right, set your timer for 2.5 minutes and go!

Swamp sparrows live in a variety of wetland habitats. Unlike most swamp sparrows, which live in freshwater habitats, the coastal-plain subspecies lives in tidal wetlands, where freshwater and seawater mix and the mud is gray rather than brown. Coastal-plain swamp sparrows differ from all other populations of swamp sparrows in having plumage that is gray brown rather than rusty brown. DNA analysis indicates several important genetic differences between swamp sparrows that inhabit tidal marshes and other subspecies of swamp sparrows. Therefore there must have been genetic-selection pressure on swamp sparrows in tidal marshes to become darker and grayer.

Select Strengthen for the statement that would, if true, most strengthen the argument, and select Weaken for the statement that would, if true, most weaken the argument. Make only two selections, one in each column.

(Note: On the real test, the answers aren’t labeled A through E; I did that to make it easier for us to discuss the answers later.)

Two-Part questions can be very much like critical reasoning questions or like quant questions; they can even be a mix of quant and verbal. This one is a fairly “pure” CR type. One little trick to figure out quickly what you’ve got is to look at the headers of the two answer columns. For this problem, the two headers are Strengthen and Weaken, so that (plus a lack of any obvious formulas, numbers, geometry figures, etc.) is a pretty clear sign that we’ve got a CR subtype, and we already know how to do those! As such, we’re going to use our 4-step CR process.

Step 1 is to identify the question type so that we know what we’re looking for. For strengthen and weaken questions, we always have to find the conclusion. If I can brainstorm any assumptions, I’ll also note those. To strengthen, we have to find an answer that makes this conclusion a little bit more likely to be valid. To weaken, we have to find an answer that makes this conclusion a little less likely to be valid.

Okay, let’s deconstruct the argument (step 2). The first column contains the argument. The second column shows my thoughts as I read that part of the argument. Below the table, I show my actual notes. Anything in [brackets] indicates my own thoughts, not something the argument actually said.

Gen diff → must have gen-sel press for color

[No alt reason for diff? Gene only?]

One of the things that I thought while I was reading the argument was that maybe the birds are different colors because the mud is different, and they roll around in the mud or something like that. The author concludes that the color MUST be due to genetic differences… so the author is also assuming that there are no other possible causes.

Step 3 is to state the goal. In this case, I’m looking for one thing that will make the argument a little more likely to be valid and another thing that will make the argument a little less likely to be valid.

On to step 4: work from wrong to right! You have a couple of options here. You might decide that you’re going to look first for whatever Strengthens and then for whatever Weakens (or vice versa). Alternatively, you can go through each answer asking yourself whether it strengthens or weakens. I used the second method myself because I think it’s pretty much the same thing that we already do on a regular CR question – we classify the 5 answers as one of three things: strengthen, weaken or slash (neither strengthen nor weaken). The three symbols are: S, W, /.

(A) None of the genetic differences that have been identified in the genomes of coastal-plain swamp sparrows and freshwater swamp sparrows affect plumage color.

Some genetic differences were found – the argument told us that – but none (zero!) of those differences have anything to do with color. Does that affect the conclusion? Yes, it makes me wonder on what evidence the author is basing the claim! The evidence given in the first place was that there were genetic differences. The author was implying that some of those differences had to do with color. Now that I know they don’t, the author’s argument isn’t sounding quite so good. This one does seem like it’s weakening the argument. Put a W label next to A on your scrap paper.

(B) Mud in tidal marshes tends to be grayish because of the presence of iron sulfide, whereas freshwater mud is browner because of the presence of iron oxide.

Oh, hey, maybe this has to do with what I was thinking before – maybe the birds are rolling around in the mud and that’s why they’re different colors. If so, that would weaken the argument because it would provide another reason, other than genetics, for the color. Hmm. Except this one only talks about why the mud is the color that it is. This choice says nothing about whether the color of the mud somehow affects the color of the birds. This one is a bit tempting, but it’s not as good as answer A. Label this one a / and eliminate.

(C) Some species of birds that live in tidal marshes do not have gray plumage.

We’re only interested in swamp sparrows. This answer choice talks about “some species of birds” in general – that doesn’t have to be referring to swamp sparrows at all. Label C a / and eliminate it.

(D) The diets of both coastal-plain and freshwater swamp sparrows can change significantly from season to season.

If the diets change, how does that affect the argument? Let’s say that the coastal-plain sparrows eat worms in winter and grubs in summer. That doesn’t tell me anything new about what color they are. Ditto for the freshwater sparrows. Label this one a / and eliminate.

(E) Baby birds of the coastal-plain subspecies and baby birds of a freshwater swamp species, all raised on an identical diet under controlled conditions, grew plumage similar in color to that of their respective parents.

Okay, baby birds of the two different subspecies at issue were raised with an identical diet. Each set ended up still having the same color as their parents. This answer is super-tricky and I almost missed the real issue the first time I read this. Remember when we said before that, if the author is right about claiming that there MUST have been some genetic thing going on, then the author is also assuming that there isn’t some other cause for the difference in color? Well, one way to strengthen a particular cause-effect conclusion is to eliminate another possible cause for the observed effect.

In this case, the observed effect is different coloring, and the cause, according to the author, was “genetic-selection pressure.” But what if eating certain kinds of foods affects the color of the birds? If that were true, that would weaken the argument. This choice is telling us that diet does NOT affect the color of the birds, so we’ve eliminate one possible weakness. That has the roundabout effect of making the conclusion a little bit stronger. Not much… but we only need to make it a little bit more likely to be valid.

The correct answers are Strengthen: E and Weaken: A.

Key Takeaways for Two-Part Analysis questions:

(1) Two-Part questions may be more quant-like or more verbal-like. Try to figure out what you’ve got before you dive in. First, glance at the column headers for the answers. If that doesn’t help enough, glance at the “question stem” (the part that says “Select Strengthen for the statement that would…).

(2) If you’ve got a CR subtype, you’re good – as long as you’ve studied CR! Do what we always do with CR; just remember that you’re going to be asked to pick two answers that do two different things.

(3) If you get a Strengthen / Weaken question, trap answers will fall into the same general categories that we already see on regular CR strengthen and weaken questions. In this case, 3 of the answers will be slashes, one will strengthen and one will weaken. Make sure not to reverse the S and W answers!

* GMATPrep® questions courtesy of the Graduate Management Admissions Council. Usage of this material does not imply endorsement by GMAC.