Just to add to the discussion of "Purpose" or "Main Idea" questions, remember that for these questions, we're looking for the answer choice with a scope that matches that of the entire passage. For example, if the passage was about hospitals and the necessary steps that need to be taken in order to efficiently run an ER, then perhaps one paragraph refers to a specific hospital as a case study.
However, it would be incorrect to say that the main idea of the passage was to "discuss Hospital X" as that is the main idea of only one paragraph. The "Main Idea" would need to be something like "to discuss the requirements of running an ER" since that is the more general focus.
For "Main Idea" questions, we need to try to see the picture that is being formed by all of the puzzle pieces, and not get distracted by the individual pieces themselves.
It's easy to do this if you write down the Purpose of the passage BEFORE reading the first question. Remember that active reading and note-taking are essential to GMAT Reading Comp success. When you see a "Main Idea" question, you can quickly refer back to your Purpose to form a prediction.
If you forgot to write down the Purpose, or you can't make a strong prediction, there are a few good places in the passage to look for the "Main Idea." Try re-reading the last few sentences of the opening paragraph. Does the author include a thesis? Does he express a strong point of view about the topic? Another good place to look is the concluding paragraph. Does the author re-iterate a main purpose here? What is he summarizing? Focus especially on the first and final sentences.
I always, always write down my own Prediction before reading the answer choices.
