GMAT Critical Reasoning Inference Questions, Part 1
July 11, 2012
You might guess that I’m on a diet, perhaps, or maybe that I’m lactose intolerant. Or maybe it’s not the milk that’s the problem; I could be deathly allergic to chocolate. Or, you might infer (correctly) that I just don’t like the flavor.
What could you infer if the GMAT told you that I don’t eat chocolate ice cream?
You can infer that if I eat ice cream, I will always choose a flavor other than chocolate. And that’s about it.
The Inference category of GMAT Critical Reasoning questions asks you to make logically supported inferences. You take the text of the stimulus at its word (recognize these questions by language such as “If the statements above are true”), and find the answer choice that must be true on the basis of the prompt.
In your GMAT prep, you will find that the biggest challenge to solving Inference questions is that there are lots of things that could be true. Sometimes, you can cleverly piece together a puzzle and make a solid prediction. But unlike argument-based Assumption questions, Inference Q’s don’t always lend themselves to knowing the answer before you look at the choices. For instance, if I don’t eat chocolate ice cream, you can infer that I wouldn’t eat chocolate ice cream cake (which contains chocolate ice cream), that a friend who knows my dietary preferences wouldn’t buy me a scoop of chocolate ice cream (which I wouldn’t eat), and that I am more likely to be seen eating vanilla ice cream than eating chocolate ice cream (because I may or may not eat vanilla ice cream, but I certainly will not eat chocolate ice cream). Since any of these would be an acceptable answer, but only one can appear in the answer choices, trying to pin down the right answer without looking at the choices can be inefficient. Inference questions are the only CR question type where you should plan to go through all five answer choices looking for one that sounds good.
But be aware of out-of-scope traps. You have to go by what the text tells you, and nothing else. And you must be able to determine the correct answer with certainty. In the chocolate ice cream example, you don’t know if the “chocolate” or the “ice cream” is the reason that I don’t eat chocolate ice cream (or something else entirely!). You might guess that I prefer vanilla ice cream, but maybe I can’t digest the milk in any type of ice cream. You might suppose that I don’t like chocolate, but it’s possible that chocolate only tastes bad to me in ice cream form and I’m fine with chocolate bars and chocolate chips. These are the types of reasonable suppositions you might make in real life, but not the type of Inference that the GMAT requires you to make.
A new electronic security system will only allow a single person at a time to pass
through a secure door. A computer decides whether or not to unlock a secure door
on the basis of visual clues, which it uses to identify people with proper clearance.
The shape of the head, the shape and color of the eyes, the shape and color of the
lips, and other characteristics of a person’s head and face are analyzed to determine
his or her identity. Only if the person trying to open a secure door has the required
clearance will the door unlock. Because this new system never fails, an unauthorized
person can never enter a secure door equipped with the system.
If the statements above are true, which of the following conclusions can be most
(A) The new system is sure to be enormously successful and revolutionize the
entire security industry.
(B) The new system can differentiate between people who are seeking to open a
secure door and people passing by a secure door.
(C) No two people have any facial features that are identical, for example,
(D) High costs will not make the new security system economically unviable.
(E) The new computer system is able to identify some slight facial differences
between people who look very similar, such as identical twins.
Step 1: Identify the Question Type
Since the stem asks us to accept the statements as true
and draw a conclusion on the basis of them, this is an
Step 2: Untangle the Stimulus
The stimulus tells us that a new electronic security system
is completely failsafe and will never allow an unauthorized
person through a door equipped with the system. And the
system allows an authorized person to enter solely on the
basis of the person’s appearance and facial features.
Step 3: Predict the Answer
Attempting to predict the correct inference could waste
time, but on the GMAT, to make an inference means to
determine what must be true, not just what could or might
be true. It’s crucial to approach the answer choices with
this in mind.
Step 4: Evaluate the Choices
(A) is out of scope. We have no evidence of how the
security industry is going to respond to the new system.
(B) doesn’t need to be true. The new system doesn’t need
to differentiate between people passing by the door and
people trying to enter, as long as it lets authorized people
in and keeps unauthorized people out.
(C) is too extreme.
We don’t know that any one feature cannot be the same.
All we know is that all of the features can’t be the same.
According to the stimulus, the security system examines
multiple facial features to determine identity.
(D), costs are outside the scope of this stimulus,
since the stimulus only discusses the likelihood that
unauthorized people will be able to get past the security
system and through a secure door.
(E) If one twin is authorized and the
other isn’t, we know the door must be able to tell them
apart, because the stimulus tells us that the security
system never fails. Thus, (E) must be true.