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Retrieval-Augmented Summarisation

Overview

Retrieval-Augmented Summarisation combines two processes:
  1. Retrieval: Find the most relevant documents or document chunks.
  2. Summarisation: Generate a concise summary using the retrieved information.
Instead of sending all available documents to a language model, the system first retrieves the most relevant content and then asks the model to create a summary.

1. Why Retrieval-Augmented Summarisation?

Traditional summarisation usually works like this:
This can become difficult when:
  • There are many documents.
  • Documents are too large.
  • Only specific information is needed.
  • The context exceeds the model’s context window.
  • Irrelevant information affects the summary.
Retrieval-Augmented Summarisation improves this process:
The model receives only relevant information.

2. Basic Workflow

The complete process has two main stages.

Stage 1: Retrieval

Example query:
The retriever might return:

Stage 2: Summarisation

The retrieved documents are passed to the language model.
Example:

3. Core Components

A Retrieval-Augmented Summarisation system typically contains: The architecture is:

4. Summarisation With Context From Retrieved Documents

The main idea is to give the model a selected context. Suppose the knowledge base contains 1,000 documents. A query is:
Instead of sending all 1,000 documents:
the system retrieves only the most relevant documents:
This makes the summarisation:
  • More focused
  • More efficient
  • More relevant to the query
  • Less likely to include unrelated information

5. Building the Context

Retrieved documents are usually combined into a single context string. Example:
Suppose three documents are retrieved:
The resulting context becomes:
This context is then passed to the summarisation prompt.

6. Prompt Design for Summarisation

Prompt design is important because it determines:
  • What information should be included.
  • How long the summary should be.
  • What style should be used.
  • Whether unsupported information should be avoided.
A basic summarisation prompt:
A better prompt:
This reduces the chance of the model generating information outside the retrieved context.

7. Query-Focused Summarisation

A useful type of Retrieval-Augmented Summarisation is query-focused summarisation. The summary is generated based on a specific query. Example:
The query guides both:

8. Generic vs Query-Focused Summarisation

Generic Summarisation

The system summarises the retrieved content without a specific focus.
Prompt:

Query-Focused Summarisation

The query determines what should be included in the summary.
Prompt:
Query-focused summarisation is particularly useful in RAG systems.

9. Important Prompt Parameters

A summarisation prompt can define several constraints.

Summary Length

or:

Output Format

Example:

Grounding

This helps reduce hallucinations.

Missing Information Handling

This prevents the model from inventing missing details.

10. A Strong Summarisation Prompt

A more complete prompt can be:
This prompt includes four important instructions:

11. Retrieval Quality Affects Summary Quality

The summarisation model can only summarise the information it receives.
But:
This is often described as:
Even a powerful LLM cannot reliably generate a good summary if the relevant information was never retrieved.

12. Important Retrieval Parameters

Top-k

The value of k determines how many documents are retrieved.
The appropriate value depends on the document size and task.
Documents are retrieved based on semantic similarity.

Chunk Size

Large documents are usually split before indexing.
If chunks are too large:
  • Retrieval may be less precise.
  • More irrelevant information is included.
If chunks are too small:
  • Important context may be split across multiple chunks.
Chunk size is therefore an important design decision.

13. Chunk Overlap

Chunk overlap keeps some text shared between consecutive chunks. Example:
The overlapping content is:
Overlap helps preserve context when important information appears near chunk boundaries.

14. Map-Reduce Summarisation

When the retrieved context is too large for a single model input, a common approach is map-reduce summarisation.
This has two stages.

Map Stage

Each document or chunk is summarised independently.

Reduce Stage

All partial summaries are combined.

15. Refine Summarisation

Another strategy is refine summarisation. The model starts with an initial summary.
Conceptually:
This is useful when processing documents sequentially.

16. Stuff Summarisation

The simplest approach is sometimes called stuffing. All retrieved content is inserted directly into one prompt.
Example:
This approach works well when the context is small enough to fit into the model’s input limit.

17. Comparison of Summarisation Strategies


18. Retrieval-Augmented Summarisation Pipeline

The complete pipeline can be represented as:

19. Key Concepts


Summary

Retrieval-Augmented Summarisation extends the RAG workflow by retrieving relevant information before generating a summary.
The two most important factors are:
A good summarisation prompt should clearly define:
This approach is useful when working with large collections of documents where only the information relevant to a particular topic or query needs to be summarised.