面试题库

14 天 RAG：从检索到可信回答

D8 评估先行：搭 golden set、算召回与排序指标、用模型当裁判判忠实度

• RAG 的评估集里为什么一定要放语料里没有答案的问题？不放会掩盖什么？Why must a RAG evaluation set include questions the corpus cannot answer, and what does leaving them out hide?

  国内高频海外高频基础#evaluation#abstention#golden-set

  分析过程 · 先想清楚再作答

  1. 这题看着简单，实际是在问「你有没有想过评估集本身也会说谎」。答成「为了测试拒答功能」只算及格，答出「不放会让某个故障在报表上完全不可见」才是满分。
  2. 推导只有一步：一个只会硬答的系统，在只有可答问题的评估集上能拿到很高的分——它每次都塞材料给模型，模型每次都编一段话，而评估集根本没有「应该拒答」这一栏。于是最危险的故障在报表上是不存在的。
  3. 结论：无答案问题是唯一能让「乱编」显形的东西。它不参与召回率，它的指标是拒答率——检索侧有没有把不够格的候选全挡下来，生成侧有没有真的说出「资料里没有」。
  4. 出题上有个必须说的细节：无答案问题必须留强干扰词，比如问「网页端支持哪些浏览器」而语料里恰好有一句「提交工单请附上浏览器与版本」。没有干扰词的无答案题检索器一条都捞不到，你测出来的是分词器不是系统。
  5. 可预期的追问是「拒答率低怎么办」。分两层查：先看检索侧的门槛是不是形同虚设（分数阈值定得太低，不相干的块也过关），再看生成侧的提示词有没有明确的拒答指令，两层都要有，只靠提示词兜是不牢的。

  How to reason about it · think before answering

  1. It looks easy but really asks whether you have considered that the eval set itself can lie. 'To test the refusal path' is a pass; 'without them the worst failure is invisible in the report' is a full mark.
  2. The derivation is one step: a system that always answers scores well on a set of answerable questions only. It stuffs context in, the model writes something, and the set has no column for 'should have refused'. The most dangerous failure simply does not appear.
  3. Conclusion: unanswerable questions are the only thing that makes fabrication visible. They are excluded from recall and scored on abstention instead - did retrieval gate out every weak candidate, and did generation actually say the material does not cover this.
  4. One authoring detail worth stating: unanswerable questions need strong distractor terms. Ask which browsers the web client supports when the corpus only says 'attach your browser and version when filing a ticket'. Without distractors retrieval returns nothing and you are testing your tokenizer, not your system.
  5. Expected follow-up: what if the abstention rate is low? Check two layers - whether the retrieval score gate is effectively a no-op, and whether the generation prompt carries an explicit refusal instruction. You need both; a prompt alone is not a reliable gate.

  答题要点

  • 只有可答问题的评估集，会让「不知道也硬答」这个故障完全不可见。
  • 无答案问题不算召回率，它的指标是拒答率，检索侧和生成侧各看一层。
  • 出题必须留强干扰词，否则检索器一条都捞不到，测的是分词器。
  • 建议无答案题占比不低于评估集的一成五，跟多跳题一起构成覆盖度底线。
  • 拒答率低要分两层查：检索门槛是否形同虚设，生成提示词有没有拒答指令。

  Key points

  • An all-answerable eval set makes 'answers confidently when it should not' completely invisible.
  • Unanswerable items are scored on abstention, not recall, and you check both the retrieval gate and the generation refusal.
  • Author them with strong distractor terms, or retrieval returns nothing and you are testing the tokenizer.
  • Keep them at roughly 15% or more of the set, alongside multi-hop items, as the coverage floor.
  • A low abstention rate splits into two causes: a no-op retrieval score gate, or a missing refusal instruction in the prompt.