GMAT Weaken CR: Trap #1 Unlinked Analogy | OG | Salmonella

This post uses the Official GMAT question that begins "A lethal strain of Salmonella recently showed up in a European country" to explain how to evaluate analogy-based choices in Weaken questions.

Many Weaken arguments reach a conclusion about one specific situation: a particular place, group, or period. Some answer choices then bring in evidence from somewhere else, such as another place, another group, or another time. These analogy-based choices are easy to misjudge in both directions. Some students pick them because they share the argument's terms and seem to add wider evidence. Others eliminate them on sight because they are "about something else." Both are guesses.

The reliable way to judge them is the same standard every correct weakener must meet: it brings in a new piece of information that reduces your belief in the conclusion, without challenging any premise. (If you are new to this standard, this article breaks it down: What Every Correct Weakener Must Do.) For an analogy-based choice, the middle part is where the real test lies. The evidence has to change what you believe about the situation the conclusion is actually about.

The Question Setup

Let's see how this works in an example.

In this question, a microbiologist responds to investigators. The investigators blamed the severity of a Salmonella outbreak in a European country on the overuse of antibiotics, because the bacteria tested were drug resistant. The microbiologist concludes this is unlikely, since patients in that country cannot get antibiotics without a prescription and the country's doctors prescribe antibiotics less readily than doctors in any other European country.

Before looking at the choices, pin down the conclusion precisely, including its scope. It is about one country, and it claims that antibiotic overuse is unlikely to be behind the severity of this outbreak. A correct weakener must make it more believable that antibiotic overuse could have contributed to the severity of the outbreak in this country.

The Trap: Unlinked Analogy

Two choices bring in evidence from other countries.

The first says that antibiotic use in two neighboring countries has risen over the past decade. It is new information, and it does not challenge a premise. But does it reduce your belief in the conclusion? It tells you about antibiotic use next door and gives you no reason to connect that to antibiotic use here. Whether use in those countries rose, stayed flat, or fell, your belief in the microbiologist's conclusion stays exactly where it was.

The second says that drug-resistant strains of Salmonella have not been found in countries where antibiotics are not generally available. This one is more tempting, because it seems to connect antibiotics to drug resistance, the same link the investigators relied on. Run it through the standard anyway. It is new, and it does not challenge a premise. At most, it suggests that where antibiotics are not generally available, resistant strains tend not to appear. But antibiotics are available in this country, through prescriptions, and the question is not whether antibiotics are used here but whether they are overused. The choice says nothing about that, so your belief in the conclusion does not move.

This is the trap worth naming: the Unlinked Analogy. A choice brings in evidence from another situation, uses the argument's terms, and seems to belong, but it never gives you a reason to expect the same in the situation the conclusion is about. Without that link, it cannot move your belief in the conclusion.

When an Analogy Does Weaken the Conclusion

Now, for contrast, imagine a different choice (this one is hypothetical, not from the question): in a neighboring country with prescription rules just as strict, a drug-resistant Salmonella strain was traced to antibiotic overuse in livestock. This is also evidence from another country, but it works. The other country resembles this one in exactly the way the microbiologist's reasoning depends on, strict prescribing. And it shows that strict prescribing did not prevent antibiotic overuse there, or the drug resistance that followed, because the overuse happened outside prescriptions. That gives a real reason to think the same could happen here, and it lowers your belief that overuse is unlikely.

So the difference between the failing choices and the working one is not where the evidence comes from. It is whether the evidence carries over. Before you accept or eliminate an analogy-based choice, ask:

What does this choice actually claim is true in the other situation?
Is there a reason to expect the same in the situation the conclusion is about?
If so, does that reduce your belief in the conclusion?

If the answer to all three is yes, the choice is doing real work. If any one fails, you can eliminate it, and you will know exactly why.

The Correct Answer

The correct choice in this question passes the standard a different way: people in this country often eat foods from animals given antibiotic laden feed. It is about this country directly, it points to a way people here could be consuming antibiotics that the microbiologist's reasoning did not account for, and so it makes antibiotic overuse more plausible here.

The Habit This Builds

The habit worth building is to state the conclusion in your own words, including its scope, before you read the choices. Then no choice gets accepted or rejected because of where its evidence comes from, only because of what it does to the conclusion in front of you.

Part of the Weaken Beginner Series

This question is part of the Weaken Beginner Series, in which easy Official GMAT questions are used not to test what you know, but to reveal how you think. Each question targets a specific reasoning habit, one that shows up in a simple form on easy questions and a complex one on hard questions. If you are getting started with Weaken questions or if Weaken questions feel inconsistent for you, this series is built to show you exactly where the process is breaking down.

Solve the question on your own first. The reasoning you apply matters more than the answer you reach.

Traps like this one are easier to catch once you know which step in your own process lets them through.

You can find that step by taking a free CR diagnostic at gmatquizmaster.com, which comes with personalized feedback on your attempt. The Weaken Beginner Series continues with more easy Official questions, each built around one reasoning habit.

Weaken Trap #1, Unlinked Analogy: When Evidence From Elsewhere Does Not Carry Over