Main Idea and Topic

Main idea questions look like the vaguest type on the subtest and they are actually the most mechanical, because the two wrong shapes are so consistent.

Topic against main idea

They are different questions and the test asks both.

A hydraulic jack multiplies force by applying pressure across two pistons of different sizes. A small force on the narrow piston produces a large force on the wide one. The trade is distance: the narrow piston must travel far to move the wide one a little.

Topic: how a hydraulic jack works. Two or three words.

Main idea: a hydraulic jack multiplies force by trading distance for it. A full sentence, and it makes a claim.

A topic question asks "this passage is mainly about" or "the best title for this passage". A main idea question asks "the main idea is" or "the author's central point is". A title is short and neutral; a main idea is a complete thought.

Where the main idea sits

Most often in the first or last sentence, and the passage type tells you which.

• A passage that states then supports puts it first, and the rest is evidence.
• A passage that builds then concludes puts it last, often after a signal like "so", "therefore" or "the result is".
• A passage with no single sentence carrying it needs you to write it yourself, which is what the "in your own words" step is for.

Read the first and last sentences deliberately. That is where to look before you look anywhere else.

The two wrong shapes

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Every main idea question offers these.

Too narrow

A real fact from the passage, offered as the point of the whole thing.

From the jack passage above: "A small force on the narrow piston produces a large force on the wide one."

That sentence is in the passage and is true, which is what makes it attractive. It is not the main idea because it covers one sentence and ignores the trade-off the passage ends on.

Test: does the choice account for every sentence, or just one?

Too broad

A statement the passage would support but which reaches far beyond it.

From the same passage: "Machines make work easier."

True, consistent with the passage, and it would equally summarize a paragraph about levers, pulleys or gears. A main idea that would fit a different passage just as well is not this passage's main idea.

Test: could this sentence head a passage about something else entirely?

Working one through

The battery does not power a running engine. Once the engine starts, the alternator takes over, supplying the electrical system and recharging the battery at the same time. A car with a failed alternator may run for a short time on battery charge alone, then stop. A car with a dead battery but a sound alternator will keep running once it has been started.

Say the point in your own words: the alternator, not the battery, runs the electrical system once the engine is going.

Now judge four candidates:

• "Batteries eventually fail." Too broad, and barely connected.
• "A car with a failed alternator will stop after a short time." Too narrow - one sentence of four, and it omits the contrast that follows.
• "The alternator supplies the electrical system while the engine runs, and the battery only starts it." This is it. It covers the whole passage, including both of the contrasting cases.
• "Car electrical systems are complicated." Too broad, and the passage is making a specific claim rather than a general one.

The best-title question

A variant that trips people because the instinct is to pick something catchy.

A good title is the topic, not a slogan, and it covers the whole passage. For the alternator passage, "The Alternator and the Battery" is a good title. "How Cars Work" is too broad, and "What Happens When an Alternator Fails" is too narrow.

Apply the same two tests.

What you can skip

• Summarizing in your own words at length. A one-line prediction is enough to test the choices; the passages are too short to need more.

Where people lose points

Picking a true detail. It is in the passage, so it feels safe. Check whether it covers everything.

Picking a general truth. It is unarguable, which feels safe for the opposite reason. Check whether it could head a different passage.

Answering the topic when asked for the main idea. A topic has no verb.

Reading only the first sentence. It carries the point often, not always, and a passage that builds to a conclusion will mislead you badly.

Choosing a choice with an absolute the passage does not support.

Work one in under a minute

Checklists were introduced in aviation after a crash caused by a pilot forgetting a single step. They work not because pilots cannot remember, but because memory is unreliable under pressure and a written list is not. The same reasoning has since moved the practice into surgery and construction.

The main idea is that

(A) pilots have poor memories (B) checklists compensate for unreliable memory under pressure (C) a crash led to a change in aviation procedure (D) surgery has adopted aviation practices

Own words: checklists work because memory fails under pressure, and that reasoning has spread.

(B). It covers all three sentences.

(A) is a distortion - the passage explicitly denies it. (C) and (D) are each one sentence of three, so both are too narrow.

Where this leads

Telling the main idea from a detail is the same judgment that separates a valid inference from a plausible one, and it is what a purpose question is really asking.