Every Extended Essay criterion is downstream of the research question: focus, argument and evaluation are all judged against it, so a bad question caps your grade before you write a word. Test it for scope, for evidence you can actually obtain, and for a verb that demands analysis rather than description.
Why most Extended Essays fail at the research question
Four thousand words is long enough to hide a bad question for about twenty pages, and not long enough to recover from one.
The question sets your ceiling
Every EE criterion is downstream of the question. Focus is judged against it. The quality of your argument is judged by whether it answers it. Even your reflection is judged on your engagement with a process the question defined. Choose badly and you end up writing a different, smaller essay well.
Three tests
Can it be answered wrongly? If no plausible investigation could produce an answer you did not expect, you have a topic rather than a question. "The causes of the 2008 financial crisis" cannot be answered wrongly. "To what extent does the repeal of Glass-Steagall explain the failure of Lehman Brothers" can, and someone could disagree with your answer, which is what makes it arguable.
Does it fit in 4,000 words? Most failed EEs are too broad rather than too narrow. If your question could be a book, it will become a summary. Narrow it until it feels almost too small; that is usually about right.
Do the sources exist, and can you reach them? A beautiful question you cannot evidence is worse than a plain one you can. Check this before you commit, not in month three.
The verb is doing real work
"To what extent", "how far", "why", "evaluate": these set what your essay has to deliver. "To what extent" promises a weighing of factors and a judgement about their relative size; if your essay lists three causes and never ranks them, it has not answered its own question. Pick the verb deliberately and then keep the promise.
Question drift is the quiet killer
Essays evolve as you read. What starts as a question about institutional causes becomes an essay about regulation, and nobody updates the question on the title page. The examiner assesses the essay against the question you printed. Re-read your question after every major draft and ask whether the essay you now have answers that.
Use your one draft well
Your supervisor may comment on one draft and may not edit it. That single reading is the most valuable thing available to you and most students spend it on a draft that is not ready. Get the question right first, then a full argument, then spend the draft.
Testing a question before you commit? That is what an EE feedback round is for: $55, or $105 with a recheck later in the cycle. We interrogate the question before anything else.