Summary of a Haystack: A Challenge to Long-Context LLMs and RAG Systems

🌈 Abstract

The article discusses a new benchmark called "Summary of a Haystack" (SummHay) for evaluating the performance of long-context language models (LLMs) and retrieval-augmented generation (RAG) systems. The key points are:

• LLMs and RAG systems can now handle very long input contexts, but evaluating their performance on such tasks remains challenging.
• The authors propose using summarization as a testbed for evaluating long-context models, as it requires reasoning over large amounts of text and understanding the relative importance of content.
• They describe a procedure to synthetically generate "Haystacks" of documents, ensuring that specific insights repeat across documents. The SummHay task then requires a system to summarize the relevant insights and precisely cite the source documents.
• The authors evaluate 10 LLMs and 50 RAG systems on SummHay, finding that it is an open challenge for current systems, with even the best models lagging behind estimated human performance.
• The SummHay benchmark is open-sourced, and the authors hope it will drive progress towards systems that can match or surpass human performance on long-context summarization.

🙋 Q&A

[01] Introduction

1. What are the key challenges in evaluating long-context LLMs and RAG systems? The key challenges are:

• Evaluating the output quality of such systems on long-context tasks remains difficult, as tasks like Needle-in-a-Haystack lack complexity.
• Summarization can play a central role in such evaluation, but prior work has focused on short-input, single-document settings.
• Existing summarization evaluation often relies on low-quality reference summaries and automatic metrics that do not correlate well with human judgments.

2. How does the authors' proposed SummHay benchmark address these challenges? The authors address these challenges by:

• Synthetically generating "Haystacks" of documents, ensuring that specific insights repeat across documents.
• Designing the SummHay task, which requires systems to process the Haystack and generate a summary that identifies the relevant insights and precisely cites the source documents.
• Implementing a highly reproducible automatic evaluation that can score summaries on Coverage (of reference insights) and Citation (quality of document citations).

[02] Summary of a Haystack Framework

1. What are the key steps in the Haystack generation process? The key steps are:

1. Generating a list of subtopics and specific insights for each subtopic.
2. Synthesizing documents that include the selected insights, ensuring each insight appears in at least 5 documents.
3. Transforming each subtopic into a query, and instructing systems to generate a summary in bullet-point format that covers the relevant insights and cites the source documents.

2. How do the authors ensure the quality and validity of the generated Haystacks? The authors implement several verification steps to ensure:

• Subtopics are distinct and unique, with no overlap.
• Insights are specific, independent, and solely relevant to a single subtopic.
• Documents include the expected insights and do not contain extraneous insights.
• The mapping between insights and documents is sound and can be precisely traced.

3. What are the key characteristics of the SummHay benchmark?

• It is instantiated across two domains: conversations and news.
• Each Haystack contains around 100 documents, totaling approximately 100k tokens.
• There are 10 Haystacks in total, each with around 9 subtopics and 62 insights on average.

[03] Evaluation Protocol

1. How do the authors evaluate the quality of system outputs on SummHay? The authors define two key metrics:

1. Coverage Score: Measures the overlap between the system's summary bullets and the reference insights.
2. Citation Score: Measures the precision and recall of the document citations provided by the system. The Joint Score combines Coverage and Citation to provide an overall measure of performance.

2. How do the authors ensure the reproducibility of the manual evaluation? The authors had two authors and two professional annotators independently annotate a subset of 35 summaries. They found a strong inter-annotator agreement, with a correlation of 0.77 on the Coverage Score.

3. How do the authors validate the automatic evaluation using LLMs? The authors recruited annotators to annotate 200 summaries and used this data to evaluate 5 LLMs as automatic evaluators. They found that GPT-4o achieved a strong positive correlation (0.71) with human annotation, at a fraction of the cost of the most expensive model.

[04] Results

1. What are the key findings from the large-scale evaluation of LLMs and RAG systems? The key findings are:

• SummHay is challenging for all systems evaluated, with all models significantly below the estimated human performance.
• There are non-trivial trade-offs between using a RAG pipeline and a long-context LLM, with RAG systems typically improving citation quality at the cost of insight coverage.
• Using advanced RAG components (e.g., Cohere's Rerank3) leads to end-to-end performance boosts on the task.
• Long-context LLMs exhibit a position bias, favoring information at the top or bottom of the context window.

2. How do the authors estimate human performance on the SummHay task? The authors recruited two professional annotators to perform the SummHay task on a subset of the Haystacks. They found that human annotators can significantly outperform the best-performing systems, achieving a Joint Score of 56.1 compared to the best system's 44.6.

3. What are the key limitations and future directions discussed by the authors? Limitations include:

• The data synthesis process and analysis focus only on summarization relevance, and could be extended to other dimensions like coherence and factual consistency.
• The insights are focused on factoid-style information, and the task could be made more complex by including insights with more diverse overlap and disagreement across documents.

Future directions include:

• Extending the SummHay benchmark to non-English languages.
• Investigating the different failure modes of low-scoring summaries in more detail.