Why Do AI Answers Leave Out Half of Your Question?

Because a complex question is almost never a single question — it's a group of sub-questions with different weights, and no answer engine covers all of them. A study from the Georgia Institute of Technology and Salesforce AI Research (NAACL 2025) decomposed 200 complex questions into around 4,000 sub-questions and measured how many showed up in the answers of three commercial engines: You.com, Perplexity AI, and Bing Chat. All three do prioritize core sub-questions over secondary ones — but they still miss around half of those core sub-questions.

It's the criterion that carries 12% weight in Nostos: Sub-Question Coverage. It doesn't measure whether you touched the topic — it measures whether you answered every part that matters.

What a "core" sub-question is, and why it's the one that matters

It's the part of your question without which the answer stays incomplete — not an extra detail, but a necessary piece of the reasoning. The paper classifies every sub-question into three types:

• Core: central to the topic, provides the reasoning and data essential to answering the main question.
• Background: optional — gives context that helps understanding, but isn't strictly necessary to answer.
• Follow-up: not needed to answer the original question; comes up afterward, once someone already has the answer and wants to go deeper.

What they found when evaluating real engines

That all three make the right distinction: they cover core sub-questions more than background or follow-up ones. But none of them consistently retrieves all the information needed for the core ones, and even when that information was already among the cited sources, it often didn't make it into the final answer. Two more findings from the same study: core sub-question coverage predicts which of two answers a human prefers with 82% accuracy — more than directly asking a model to compare the two answers, which reaches 71%. And when a RAG system prioritizes core sub-questions during retrieval, the resulting answer beats the baseline without that step up to 74% of the time.

What you can do today

Decompose your own topic before writing, not after. Four concrete steps:

• Write down the core sub-questions first — the ones that, if missing, leave the answer incomplete.
• Give each core sub-question its own H2 or H3; don't blend them into a single paragraph.
• Use background sub-questions to fill in, not to open the article.
• Save follow-up questions for the end, or for a separate article.