Jo Kristian Bergum
Jo Kristian Bergum
Vespa Solutions Architect

Improving retrieval with LLM-as-a-judge

This post demonstrates how to create your own reusable retrieval evaluation dataset for your data and use it to assess your retrieval system’s effectiveness. A reusable relevance dataset allows for systematic evaluations of retrieval approaches, moving beyond basic Looks-Good-To-Me@10.

We demonstrate this process by building a large relevance dataset for our Vespa documentation RAG solution (read more about The TLDR of the process:

  • Build a small labeled relevance dataset by sampling queries and judging retrieved documents
  • Prompt an LLM to judge the query and document pairs, then align the LLM-as-a-judge with human preferences established by the first step.
  • Sample a large set of queries and retrieved documents using multiple methods. These are judged by the calibrated LLM-as-a-judge
  • You now have a large relevance dataset that you can use to experiment with retrieval methods or parameters tailored for your data and retrieval use case

ranking correlation

GPT-4 judgments aligned with human judgments for

It may sound straightforward, but some nuances make it a complex topic. This article has the following major parts:

  • Retrieval is short for Information Retrieval (IR)
  • Ground truths for evaluation of Information retrieval systems
  • Sampling of queries and documents to judge
  • Evaluation metrics used to evaluate IR systems
  • Towards using LLM-as-judge for scaling relevance judgments
  • Building a large relevance dataset using LLM-as-a-judge
  • Evaluation results and summary
  • FAQ

Let’s delve dive into it.

Retrieval is short for Information Retrieval (IR)

Information retrieval (IR) is an established research area where reliable evaluation of retrieval systems is central. Reusable relevance test collections (documents, queries, and relevance judgments) allow researchers to experiment with methods and parameters.

From The Philosophy of Information Retrieval Evaluation by Ellen M. Voorhees

Evaluation conferences such as TREC, CLEF, and NTCIR are modern examples of the Cranfield evaluation paradigm. In the Cranfield paradigm, researchers perform experiments on test collections to compare the relative effectiveness of different retrieval approaches. The test collections allow the researchers to control the effects of different system parameters, increasing the power and decreasing the cost of retrieval experiments as compared to user-based evaluations.

Our goal with this post is to help you build your own reusable relevance dataset that you can use to compare the relative effectiveness of retrieval systems or approaches, for your data and your type of queries.

When you are doing retrieval for augmenting an LLM prompt (RAG), you are effectively doing information retrieval. While you might be using off-the-shelf text embedding representations for this task, the implementation of retrieval extends far beyond a single cosine similarity score. A retrieval system can incorporate various techniques beyond the scope of this post, such as query rewriting, retrieval fusions, ColBERT, splade, cross-encoders, phased ranking, and more.

With a reusable dataset, you can quantify effectiveness, iterate on your approach, observe the impact of different parameters, and make informed decisions regarding the trade-offs between effectiveness and deployment cost.


Simply put, we evaluate a retrieval system’s effectiveness by measuring its ability to retrieve relevant documents for a query. The relevance of a query-document pair can be assessed by a domain expert or a relevance assessor following guidelines. By now, you might already start thinking about IR metrics with fancy names such as nDCG and MRR, we will get to that, but first, we need labeled data, or ground truths. Once we have enough labeled data, we can start measuring effectiveness, quantified by IR evaluation metrics.

Ground Truths for IR

To build any system that adds intelligence using ML, including those based on foundational LLMs, we need a sample of ground truths to evaluate the system. With more labeled data, we can fine-tune the system, but at a minimum, a ground truth dataset is essential for testing and evaluation. In the context of retrieval, ground truths typically consist of a triplet: a query, a passage, and a relevance judgment.

The relevance judgment could be graded, for example, using a scale such as irrelevant, relevant, and highly relevant. Alternatively, these grades could be converted to binary scale: irrelevant versus relevant. The judgment labels are usually converted to a numeric utility score; for instance, 0 could represent irrelevant and 1 could represent relevant.

Creating ground truths for retrieval evaluation

One simple (and boring), but effective way to produce ground truth labels is; looking at data.

Below is a screenshot of the search result page for a real user’s information need, expressed as the query can i set term weights in yql. In this case, a domain expert has judged the top three retrieved passages using graded judgments:

The first hit is labeled as highly relevant as it seemingly answers the query intent fully, with a concrete example of how to set the term weight using the Vespa YQL query language. The second document is annotated as relevant, as it covers the same topic, but using the Vespa simple query language instead of YQL. This label requires domain knowledge as the simple query language can be referenced via userQuery() in YQL.

When we use an expert to judge the results of a query that a user has typed, the expert can not reliably predict what the actual information needed was, or how satisfied the user was with these retrieved passages. The user who typed this query might have judged both hit 2 and 3 as irrelevant. Relevance is subjective.

We can add UI elements that allow the user to provide feedback (albeit sparse and noisy) on each retrieved document. Alternatively, instead of explicit feedback, many real-world retrieval systems rely on implicit feedback through user engagement signals, such as clicks or skips. There is one major problem, the above search result page displays the full paragraph, so the user does not need to click the result to determine its relevance to the query, making it hard to gather reliable labels using clicks. This is a growing concern among search practitioners. As real-world retrieval systems evolve towards answering systems, implicit user feedback signals diminish.

Sampling of queries and documents to judge

Above, we demonstrated a judgment process with a single query. However, to evaluate the effectiveness of a system, we’ll need more examples than a cherry-picked query where the current ranking system is effective. We need to select more queries to evaluate - options:

  • Sample queries from the production system
  • If we don’t yet have a production system (or users), one alternative is generating synthetic queries via prompting an LLM, but this is not a great substitute for real user queries

real queries

A screenshot of a dashboard, where we can see a sample of queries performed on Predicting the user’s underlying information need, given a short query, can be difficult.

How many queries do we need? Not that many. Most TREC collections have fewer than 100 queries, and 25 is better than one. More is better.

Sampling of documents to judge

This is a considerably harder process than sampling queries. Most of the documents in the collection will be irrelevant for any query, but we cannot be sure unless we judge every document for the query. Assuming that it takes 5 seconds to judge a (query, document) pair and that our collection is 5K documents, a single person would use 4 hours to label all the documents for a single query. This process doesn’t scale.

In practice, we need a way to sample documents for queries. One alternative is to sample from top-k rankings produced by multiple retrieval techniques to find diverse candidates for every query. This process is known as pooling in IR evaluations. Typically, at the start of a TREC conference track, the organizers will publish the document collection and the queries. Then, participants will submit rankings for the queries, forming the pool of (query, document) pairs that are candidates for the judgment process.

A bias is introduced if we only sample from one retrieval technique, like in the screenshot above, as the labels will not be helpful to evaluate other retrieval models reliably. This is because other retrieval models can surface other relevant documents for the same query and unlabeled (query, document) pairs will by default be assigned as irrelevant. This makes it difficult to compare retrieval systems purely by those labels without knowing if there were many “holes” in the judgments.

Evaluation metrics used to evaluate information retrieval systems

The way we evaluate a system or method (given the ground truth triplets) is that for each query in the ground truth set we ask the system for the top-k ranked documents, then compute the chosen metric(s). Finally, the overall score of the system is the average across all queries. You can safely skip the rest of this section if you already know P@k, R@K, and nDCG@k.

Usually, we use a cutoff k of how far into the ranked list produced by the system we compare with the ground truth. This cutoff can typically be 10 for ten blue links search applications and potentially larger for RAG applications using models with large context windows.

Given the query example from the previous section we had the following ranked list using highly relevant = 2, relevant = 1, and irrelevant = 0: [2,1,0]. We also can assume that the ideal ordering would be [2,2,1] as there is one additional highly relevant document in the ground truth.

Precision at k (P@k) measures the relevance of the top k search results. It is calculated by dividing the number of relevant results in the top k positions by k. For example, if we use a grade threshold of 2 (Highly relevant), the P@3 for the screenshot above would be 0.33. This means that out of the top 3 results, 1 is highly relevant. The position of the relevant results within the top k does not matter, so P@3 cannot distinguish between rankings [2,1,0], [2,0,1], and [0,1,2].

Recall@k reflects the proportion of relevant documents retrieved among the top k retrieved documents. In practice, knowing all the relevant documents for a given query is impossible. Recall@k does not take into account positions within k. With a relevance cutoff of 2, the recall@3 for the above screenshot would be 0.5 as only 1 out of the 2 were retrieved into the top 3 positions.

Normalized Discounted Cumulative Gain (nDCG@k) considers both the grade (gain) and the position within k, and the gain is discounted (reduced) with increasing rank position. It’s calculated as the ratio of the actual DCG@k to the ideal DCG@k. The normalization makes it possible to have a meaningful average over queries when there are differences in the number of judgments per query.

For a better explanation of nDCG (and other evaluation metrics), see this slide deck (pdf) from CS276.

Towards using LLMs as relevance judges

As human labeling quickly becomes prohibitively costly for the comprehensive evaluation of a retrieval system, using LLMs as judges becomes an attractive alternative. However, there is a significant catch: we need a method to evaluate the LLM-as-a-judge to ensure that its preferences align with human preferences.

Recent research1 2 3 4, finds that LLMs like GPT-4 can judge the relevance of passages retrieved for a query with fair correlation with human ground truths and good correlation in ranking metrics. All we need to do is to align the prompt with our human ground truths.

For, we established a human-labeled ground truth dataset of 26 queries, where passages are sampled from a single method using multiple signals. We use this dataset of 90 triplets with, on average, 3.5 human judgments per query to judge the LLM-as-a-judge.

Ground Truth

Sample of (query passage) relevance judgment triplets. We use a graded relevance scale where:

  • Irrelevant maps to 0
  • Relevant maps to 1
  • Highly relevant maps to 2

With this small dataset, which takes a few hours to gather, we ask the LLM to judge the same (query, passage) pair. In our experiments, we use GPT-4o, and the prompt is inspired by 3. As a modification, we add two static demonstrations.

def get_score(query, passage):
    prompt = f"""Given a query and a passage, you must provide a score on an
    integer scale of 0 to 2 with the following meanings:
    0 = represents that the passage is irrelevant to the query,
    1 = represents that the passage is somewhat relevant to the query,
    2 = represents that the passage is is highly relevant to the query.
    Important Instruction: Assign score 1 if the passage is
    somewhat related to the query, score 2 if
    passage is highly relevant. If none of the
    above satisfies give it score 0.
    Query: what is fast-search
    Passage: By default, Vespa does not build any posting list index structures over attribute fields. 
    Adding fast-search to the attribute definition as shown below will add an in-memory B-tree posting list structure which enables faster search for some cases 
    ##final score: 2
    Query: what is fast-search
    Passage: weakAnd inner scoring The weakAnd query operator uses the following ranking features when calculating the inner score dot product:
    Note that the number of times the term occurs in the document is not used in the inner scoring calculation.
    ##final score: 0
    Split this problem into steps:
    Consider the underlying intent of the search query.
    Measure how well the passage matches the intent of the query.
    Final score must be an integer value only.
    Do not provide any code or reasoning in result. Just provide the score.
    Query: {query}
    Passage: {passage}
    ##final score: 
    response =
        {"role": "system", "content": "You are a Relevance assessor that judges the relevance of a passage to a query."},
        {"role": "user", "content": prompt}
    result = response.choices[0].message.content
        final_score = int(result.strip())
    except ValueError:
        final_score = None
    return final_score

Details of the prompt used to judge a (query, passage) pair.

After running this for the 90 (query, passage) pairs, we have both the ground truth label assigned by the human and the GPT-4 label. With that data, we can use a confusion matrix to illustrate the differences and agreements.

Confusion Matrix

Confusion matrix comparing GPT-4 labels versus human labels

There is a fair correlation between our human ground truth labels and the GPT4 labels. For example, there are few cases where the LLM disagrees by more than one level. In only 1 case does it assign irrelevant for something that the human assigned as highly relevant, and the other way around.

We can also compare the GPT-4 labels to the human labels by evaluating different retrieval techniques for the queries. Then, evaluate the ranking metric nDCG using the human and GPT labels.

Ranking correlation

Nine retrieval methods represented in Vespa. Each point is the aggregated nDCG@10 score over all 26 queries using a retrieval method evaluated by human or GPT labels.

As we can see from the above scatter plot, there is a good correlation in the rankings. This is encouraging, as it means that we can now start expanding our dataset with more queries, sample more documents and let GPT be the relevance judge as it’s ranking the documents in a similar way to a human.

Creating our LLM-judged relevance test collection

With good correlation between the human and GPT rankings, we can gather more queries and documents for queries to create a larger collection of judgments. We sample 386 unique queries performed over the last month on For passages, we sample the top-10 rankings using six different methods.

We remove duplicate pairs (query, passage) which is the case if different methods return the same passage in the top-k rankings for the same query. After this step we are left with 10,372 unique (query, passage) pairs that we let GPT label.

Dataset statistics

  • Unique queries: 386
  • Unique documents: 3014 (with at least one judgment)
  • Average judgments per query: 26.9

Label distribution

  • Irrelevant 4817
  • Relevant 4642
  • Highly Relevant 913

This relevance collection allows us to change ranking-related parameters. We will not detail each of the methods compared here. We use nDCG@10 as the primary metric since we use graded relevance labels. We also use Judged@10, which gives information about whether there are holes in the judgments. A high judged@10 would mean that all the hits in the top-ranked positions have judgments (relevant or irrelevant).

ndcg The nDCG@10 for the different retrieval methods evaluated by GPT-4 judged The Judged@10 for the different retrieval methods evaluated by GPT-4 recall The Recall@10 for the different retrieval methods evaluated by GPT-4

We use cutoff 2 (Highly Relevant) for Recall@10. Note that all these metrics are averaged over all queries in the test collection. With this, we can start evaluating the impact of changing parameters. Furthermore, it’s also possible to use this dataset to train models using traditional learning to rank techniques. If we choose to do that, we also should build another test set that we can use to evaluate the model.


Moving towards LLM-as-a-judge for relevance assessment is an exciting direction that can enable quicker iterations (feedback loops) and eventually improved search relevancy for your data. This is important because traditional methods of evaluating retrieval systems, such as using human experts, can be costly and time-consuming. Additionally, when moving from retrieval engines to answer engines, implicit click feedback signals require that we find different (scalable) ways to obtain high-quality relevancy feedback.

By using LLM-as-a-judge, it’s possible to quickly and efficiently evaluate different retrieval methods and parameters, leading to more informed decisions about relative effectiveness and production deployment costs. It’s important to note that we need a human in the loop to ensure that we judge the LLM-as-a-judge quality so that the rankings align.


Why didn’t you try more LLMs to have more judges, like an ensemble of judges?

That is certainly an option, to use majority voting for example. We found good correlation in the ranking order with one LLM so we didn’t experiment further.

Isn’t Recall@K a better metric than nDCG@10 in the context of RAG?

The argument is that the order within the context window doesn’t matter so we should only care about whether we can retrieve all the relevant information within the K first positions where K is determined by the context window size budget. For we do display the retrieved passages and instruct the LLM to cite the sources. If the retriever places the relevant information higher up, (lower rank), then there is less scrolling to verify the generated answer. So, in short, we still want to have a precision-oriented metric that handles both a graded scale and position.

Any suggestions on how to overcome the sampling biases?

As the cost of using the LLM as the judge is a fraction of the human cost, it’s now possible to evaluate more documents per query, including also evaluating the holes (missing judgments)4.

This is useful when we introduce new retrieval methods, or; the collection changes over time.

Can I use your test collection to evaluate my own retrieval system?

The main point of this post was to encourage users to create their own dataset, for your own data and retrieval use case. It doesn’t need to be fancy. Combine with a tsv file and a scraping routine, and you can get started.

When you write about retrieval systems, what do you mean by that? A database, or a search engine, or a vector database?

All of those could be used as a retrieval system, even grep could be a retrieval system. But, you will likely be better off using a system that was built for effective information retrieval - a search engine like where you can represent many different retrieval and ranking methods.

Do you think SPLADE works better than BM25 for my use case?

This is the type of question we have been getting over the last few years as we have added new and more advanced techniques to Vespa. We hope that with this post, you can start building your own test collections and answer these questions for your data and queries.

What about learning to rank where you try to optimize the ranking, how does it relate to LLM judges?

As demonstrated in this post, the LLMs can be used to judge the relevance. Therefore, we could use the label as the target when training a model using learning to rank.

What does this mean for ranking, shouldn’t we just use the LLM for the ranking task as well?

Ultimately yes, one of the reasons we deeply integrate generative LLMs. We envision that it can improve lots of search-related tasks:

  • Query and document “understanding”
  • Categorization and classification
  • Ranking (and retrieval)

The biggest challenge is high computational complexity or cost, but we are sure the price will continue to drop over the next few years. With some of the techniques explored in this post, one can optimize ranking without significant cost by distilling the knowledge of the most powerful LLMs. We let them judge and rate the results, then distill that knowledge into fine-tuned parameters to approximate their “optimal” ranking order.