RAG Evaluation

RAG Evaluation & Testing

Measure what matters: retrieval quality, answer faithfulness, and end-to-end performance.

Key Metrics Overview

RAGAS Framework

RAGAS (Retrieval Augmented Generation Assessment) is the standard for RAG evaluation.

Installation & Setup

pip install ragas langchain_openai

from ragas import evaluate
from ragas.metrics import (
    faithfulness,
    answer_relevancy,
    context_precision,
    context_recall,
    answer_correctness
)
from datasets import Dataset

Creating Evaluation Dataset

def create_eval_dataset(test_cases: list[dict]) -> Dataset:
    """
    Create evaluation dataset from test cases.

    Each test case needs:
    - question: The query
    - answer: Generated answer from your RAG
    - contexts: Retrieved documents (list of strings)
    - ground_truth: Expected correct answer (for some metrics)
    """
    return Dataset.from_dict({
        "question": [tc["question"] for tc in test_cases],
        "answer": [tc["answer"] for tc in test_cases],
        "contexts": [tc["contexts"] for tc in test_cases],
        "ground_truth": [tc.get("ground_truth", "") for tc in test_cases]
    })

# Example test cases
test_cases = [
    {
        "question": "What is the return policy?",
        "answer": "Items can be returned within 30 days with receipt.",
        "contexts": [
            "Our return policy allows returns within 30 days of purchase. A receipt is required.",
            "Refunds are processed within 5-7 business days."
        ],
        "ground_truth": "30-day return policy with receipt required"
    },
    # Add more test cases...
]

eval_dataset = create_eval_dataset(test_cases)

Running Evaluation

from ragas import evaluate
from ragas.metrics import (
    faithfulness,
    answer_relevancy,
    context_precision,
    context_recall
)

# Run evaluation
results = evaluate(
    eval_dataset,
    metrics=[
        faithfulness,
        answer_relevancy,
        context_precision,
        context_recall
    ],
    llm=ChatOpenAI(model="gpt-4"),
    embeddings=OpenAIEmbeddings()
)

# View results
print(results)
# {'faithfulness': 0.92, 'answer_relevancy': 0.88, ...}

# Detailed per-question results
df = results.to_pandas()
print(df)

Understanding Each Metric

Faithfulness (Is answer grounded?)

Measures if the answer can be inferred from the retrieved context.

from ragas.metrics import faithfulness

# Faithfulness checks:
# 1. Extracts claims from the answer
# 2. Verifies each claim against context
# 3. Score = supported claims / total claims

# Example:
# Answer: "The product costs $99 and ships free"
# Context: "Product price: $99. Standard shipping: $5"
# Faithfulness: 0.5 (only price claim is supported)

Context Precision (Are retrieved docs relevant?)

Measures if retrieved documents are actually relevant to the question.

from ragas.metrics import context_precision

# High precision: Retrieved docs are all relevant
# Low precision: Retrieved docs contain irrelevant noise

# Scored by checking if each context chunk is needed
# to answer the question

Context Recall (Did we get everything?)

Measures if all information needed to answer is in the retrieved context.

from ragas.metrics import context_recall

# Requires ground_truth answer
# Checks if context contains info needed for ground truth

# High recall: Context has everything needed
# Low recall: Missing relevant documents

Answer Relevancy (Does it answer the question?)

Measures how well the answer addresses the original question.

from ragas.metrics import answer_relevancy

# Generates questions from the answer
# Compares generated questions to original
# Higher similarity = more relevant answer

Custom Evaluation Metrics

Retrieval Hit Rate

def calculate_hit_rate(
    test_cases: list[dict],
    retriever,
    k: int = 5
) -> float:
    """
    Calculate retrieval hit rate.
    Hit = relevant doc in top-k results.
    """
    hits = 0
    for tc in test_cases:
        results = retriever.invoke(tc["question"])[:k]
        retrieved_texts = [r.page_content for r in results]

        # Check if any relevant doc was retrieved
        for relevant_doc in tc["relevant_docs"]:
            if any(relevant_doc in text for text in retrieved_texts):
                hits += 1
                break

    return hits / len(test_cases)

Mean Reciprocal Rank (MRR)

def calculate_mrr(
    test_cases: list[dict],
    retriever
) -> float:
    """
    Mean Reciprocal Rank.
    Higher = relevant docs appear earlier in results.
    """
    reciprocal_ranks = []

    for tc in test_cases:
        results = retriever.invoke(tc["question"])
        retrieved_texts = [r.page_content for r in results]

        # Find rank of first relevant doc
        for rank, text in enumerate(retrieved_texts, 1):
            if any(rel in text for rel in tc["relevant_docs"]):
                reciprocal_ranks.append(1 / rank)
                break
        else:
            reciprocal_ranks.append(0)

    return sum(reciprocal_ranks) / len(reciprocal_ranks)

Latency Tracking

import time
from dataclasses import dataclass

@dataclass
class RAGMetrics:
    retrieval_latency_ms: float
    generation_latency_ms: float
    total_latency_ms: float
    num_docs_retrieved: int
    num_tokens_generated: int

def measure_rag_performance(rag_chain, query: str) -> RAGMetrics:
    """Measure RAG pipeline performance."""

    # Measure retrieval
    start = time.perf_counter()
    docs = rag_chain.retriever.invoke(query)
    retrieval_time = (time.perf_counter() - start) * 1000

    # Measure generation
    start = time.perf_counter()
    response = rag_chain.generate(query, docs)
    generation_time = (time.perf_counter() - start) * 1000

    return RAGMetrics(
        retrieval_latency_ms=retrieval_time,
        generation_latency_ms=generation_time,
        total_latency_ms=retrieval_time + generation_time,
        num_docs_retrieved=len(docs),
        num_tokens_generated=len(response.split())
    )

Building a Test Suite

class RAGTestSuite:
    """Comprehensive RAG test suite."""

    def __init__(self, rag_chain, test_cases: list[dict]):
        self.rag = rag_chain
        self.test_cases = test_cases

    def run_all(self) -> dict:
        """Run all evaluation metrics."""
        results = {
            "ragas_metrics": self._run_ragas(),
            "retrieval_metrics": self._run_retrieval_metrics(),
            "latency_metrics": self._run_latency_metrics(),
            "failure_analysis": self._analyze_failures()
        }
        return results

    def _run_ragas(self) -> dict:
        # Generate answers for test cases
        eval_data = []
        for tc in self.test_cases:
            answer = self.rag.query(tc["question"])
            contexts = self.rag.retriever.invoke(tc["question"])
            eval_data.append({
                "question": tc["question"],
                "answer": answer,
                "contexts": [c.page_content for c in contexts],
                "ground_truth": tc.get("expected_answer", "")
            })

        dataset = Dataset.from_dict({
            "question": [d["question"] for d in eval_data],
            "answer": [d["answer"] for d in eval_data],
            "contexts": [d["contexts"] for d in eval_data],
            "ground_truth": [d["ground_truth"] for d in eval_data]
        })

        return evaluate(dataset, metrics=[
            faithfulness, answer_relevancy,
            context_precision, context_recall
        ])

    def _run_retrieval_metrics(self) -> dict:
        return {
            "hit_rate@5": calculate_hit_rate(self.test_cases, self.rag.retriever, k=5),
            "hit_rate@10": calculate_hit_rate(self.test_cases, self.rag.retriever, k=10),
            "mrr": calculate_mrr(self.test_cases, self.rag.retriever)
        }

    def _run_latency_metrics(self) -> dict:
        latencies = []
        for tc in self.test_cases[:10]:  # Sample for latency
            metrics = measure_rag_performance(self.rag, tc["question"])
            latencies.append(metrics)

        return {
            "avg_total_ms": sum(l.total_latency_ms for l in latencies) / len(latencies),
            "avg_retrieval_ms": sum(l.retrieval_latency_ms for l in latencies) / len(latencies),
            "p95_total_ms": sorted([l.total_latency_ms for l in latencies])[int(len(latencies) * 0.95)]
        }

    def _analyze_failures(self) -> list[dict]:
        """Identify and categorize failures."""
        failures = []
        for tc in self.test_cases:
            answer = self.rag.query(tc["question"])
            if tc.get("expected_answer") and tc["expected_answer"].lower() not in answer.lower():
                failures.append({
                    "question": tc["question"],
                    "expected": tc["expected_answer"],
                    "actual": answer,
                    "failure_type": self._classify_failure(tc, answer)
                })
        return failures

    def _classify_failure(self, tc: dict, answer: str) -> str:
        """Classify failure type."""
        contexts = self.rag.retriever.invoke(tc["question"])
        context_text = " ".join([c.page_content for c in contexts])

        if tc.get("expected_answer") and tc["expected_answer"] in context_text:
            return "generation_failure"  # Right docs, wrong answer
        else:
            return "retrieval_failure"  # Wrong docs retrieved

Continuous Evaluation

# In production, log and monitor
import logging

logger = logging.getLogger("rag_eval")

def monitored_rag_query(rag, query: str) -> dict:
    """RAG query with monitoring."""
    start = time.perf_counter()

    # Get results
    docs = rag.retriever.invoke(query)
    answer = rag.generate(query, docs)

    # Log metrics
    logger.info({
        "query": query,
        "num_docs": len(docs),
        "latency_ms": (time.perf_counter() - start) * 1000,
        "answer_length": len(answer)
    })

    return {"answer": answer, "sources": docs}

Best Practices

1. Build golden dataset - 50-100 human-validated Q&A pairs
2. Test edge cases - Empty results, ambiguous queries, out-of-domain
3. Automate in CI/CD - Run eval on every change
4. Track over time - Monitor metric trends
5. Segment by query type - Different metrics for different use cases