The Praxis Core will expect that test takers can find the main idea of a passage. The most common — and the most straightforward — type of short-passage question is a “main idea” question.
The wording of the question is usually along the lines of “The main idea of the passage is … ” or “The primary purpose of the passage is … ”, and your mission is to select the answer choice that best completes the sentence. Basically, phrases like “main idea” and “primary purpose” are just fancier ways of asking
“What’s this about?”
“What’s the point of this?”
“What is this paragraph trying to say?”
Although such questions may seem easy after you’ve had a bit of practice with them, they can be difficult in the sense of being deceptively simple if you’re not used to them. Questions like these try to trick you by making the wrong answers flashier or more attractive than the right one. They may have more details in them, or they may contain more exact words from the passage.
However, you’re not looking for the statement about the passage that is the most detailed or the most specific — you’re looking for the statement that is true. And there will be only one of those. The other four choices, for whatever reason, will be wrong. A common trick that the test-writers use on such questions is to make the right answer so vague or uninteresting that you barely notice it.
The wrong answers stand out more. But never forget that all you’re trying to do is pick the statement about the purpose of the passage that’s true (in other words, not wrong) — no more and no less. Consider the following example question:
Anyone who paid attention in grade-school science class could tell you that the five classes of vertebrates are mammals, reptiles, amphibians, birds, and fish. For centuries, these categories made sense to scientists because they represented clear distinctions based on what we were able to observe about the animal kingdom.
But now that we know more about evolutionary history, the borders between these traditional and visually “obvious” classes are not so clear. A crocodile looks more like a turtle than a penguin, but the common ancestor of the crocodile and the penguin actually lived more recently than did the common ancestor of the crocodile and the turtle.
The primary purpose of the passage is to
(A) explain how penguins evolved from crocodiles.
(B) dispute some recent theories in the field of evolutionary biology.
(C) correct a misconception common in grade-school science curricula.
(D) discuss how a biological concept is more complicated than it looks.
(E) summarize a disagreement about vertebrates between two schools of zoologists.
The correct answer is Choice (D). To understand why Choice (D) is correct, consider: Does the passage “discuss how a biological concept is more complicated than it looks?” Yes, it does. In fact, it indisputably does — in other words, there is no reasonable way to argue that the passage does not do this. So (D) is the right answer because it cannot possibly be wrong.
As for the others, by now you’ve figured out that they’re all wrong. But look at what they have in common: All the wrong answers stand out by repeating specifics or key words from the passage — but they also twist those specifics so the statements are no longer true.
Choice (A) is wrong because the passage technically doesn’t say that penguins evolved from crocodiles; it says that penguins and crocodiles have a common ancestor. And even if the passage did say this, it wouldn’t be the primary purpose of the passage, because it’s only one example given right at the end.
The test-writers know that the example given at the end will be fresh in your mind, so they try to get you to jump the gun by making it the first choice!
Choice (B) is wrong because of the verb it uses. The author is indeed talking about “recent theories in the field of evolutionary biology,” but he isn’t disputing them, only explaining them (there is no indication that the author disagrees). Always remember that it only takes one false move to make an answer choice wrong!
Choice (C) is wrong because the passage technically never says that the vertebrate classes as explained in schools are incorrect (a “misconception”). The five classes of vertebrates are still mammals, amphibians, reptiles, birds, and fish. The passage discusses some interesting information that seems as if it might lead to scientists changing those categories somehow in the future, but it never says they’ve done so already!
As for Choice (E), did the passage say or imply anything about “two schools” of zoologists? No. The passage explains that zoologists (biologists that specifically study animals) nowadays have more information than zoologists did in the past, but it never hints at anything about a debate.