How to find the main idea and supporting evidence

Topic, main idea and supporting details

Three terms get mixed up, so it helps to separate them:

A quick check: the topic answers "What is this about?" The main idea answers "What is the author saying about it?" Details answer "How do I know?"

• Topic: what the passage is about, in a word or short phrase. "Sleep and teenagers."
• Main idea: the author's central point about the topic, as a complete sentence. "Most teenagers do not get enough sleep, and it affects how well they learn."
• Supporting details: the facts, examples, reasons and explanations that prove or explain the main idea. "Teens need roughly 8 to 10 hours a night" is a detail.

Five steps to find the main idea

This routine works for a paragraph, an article or a textbook section:

• Preview: read the title, headings, pictures and captions. They usually name the topic.
• Read the first and last sentences of each paragraph closely. Authors often state the main idea there, though not always.
• Name the topic in a few words.
• Ask what the author keeps saying about that topic. Look for repeated words and ideas.
• Write the main idea as one sentence in your own words, then test it: do most details support it? If a sentence explains only one paragraph, it is probably a detail.

Worked example: a stated main idea

Read this short passage: "Honeybees do far more than make honey. As they move from flower to flower, they carry pollen that plants need to produce fruit and seeds. Crops such as almonds, apples and blueberries depend heavily on bee pollination. When bee colonies shrink, farmers often have to rent hives to keep their harvests healthy."

Topic: honeybees. The first sentence hints at the point, and every later sentence explains it. Main idea: "Honeybees are important because they pollinate many of the crops people eat." Supporting details: they carry pollen between flowers; almonds, apples and blueberries depend on them; farmers rent hives when colonies shrink.

Notice that "Honeybees make honey" is true but is not the main idea. The author mentions it only to move past it.

Worked example: an implied main idea

Sometimes no sentence states the main idea, and you have to infer it from the details. Read this: "In 1900, about 4 in 10 American workers worked on farms. Tractors and combine harvesters spread through the early twentieth century. New fertilizers and crop varieties raised the harvest from each acre. Today, fewer than 2 in 100 American workers are farmers."

No sentence says what all this means. Add the details together: farm work fell sharply while machines and science made each farmer more productive. Implied main idea: "Machines and scientific advances let far fewer people grow America's food during the twentieth century."

A good inferred main idea is specific enough to fit this passage but broad enough to cover all its details. "Farming changed" is too vague. "Tractors were invented" is too narrow.

Main idea in a longer text: the Gettysburg Address

In a longer text, find the main idea of each section, then ask what they add up to. Abraham Lincoln's Gettysburg Address, given on November 19, 1863, has three short parts: the nation was founded on liberty and equality; the Civil War is testing whether such a nation can last, and the crowd has come to dedicate a cemetery; the living must finish the work the soldiers died for.

Put together, the main idea is roughly: "The best way to honor the soldiers who died is to keep fighting so that a nation based on liberty, equality and government by the people survives." The phrase "government of the people, by the people, for the people, shall not perish from the earth" is strong supporting evidence, because it states that goal in Lincoln's own words.

Choosing the best supporting evidence

Questions on tests often ask which detail best supports the main idea. The strongest evidence is directly about the main idea, specific (a number, a named example, a quotation), and from a reliable part of the text rather than a side comment.

When you write about a text, introduce evidence, quote or paraphrase it, and then explain it: "The author notes that almonds depend heavily on bee pollination, which shows that losing bees would affect real foods on our tables." The explanation is what connects the detail to the main idea.

Common mistakes

Watch for these traps, especially in multiple-choice questions:

• Choosing an interesting detail instead of the main idea.
• Choosing the topic (a phrase) when the question wants a full idea.
• Picking an answer that is too broad, such as "Animals are important," for a passage only about bees.
• Assuming the first sentence is always the main idea. Sometimes it is a hook or an opposing view the author goes on to reject.
• Adding your own opinion. The main idea is what the author says, not what you think.

How to practise and check your answer

After reading any article, try to summarize it in one sentence of no more than 20 words, then list three details that support that sentence. If you cannot find three, your sentence may be a detail rather than the main idea.

Another check is the headline test: write a headline for the passage. A good headline is close to the main idea. Newspaper articles, science magazines and textbook sections are all good practice, and comparing your sentence with a classmate's or a parent's quickly shows when one of you picked a detail.