name: llm-evaluation description: Implement comprehensive evaluation strategies for LLM applications using automated metrics, human feedback, and benchmarking. Use when testing LLM performance, measuring AI application quality, or establishing evaluation frameworks.

LLM Evaluation

Master comprehensive evaluation strategies for LLM applications, from automated metrics to human evaluation and A/B testing.

When to Use This Skill

Measuring LLM application performance systematically
Comparing different models or prompts
Detecting performance regressions before deployment
Validating improvements from prompt changes
Building confidence in production systems
Establishing baselines and tracking progress over time
Debugging unexpected model behavior

Core Evaluation Types

1. Automated Metrics

Fast, repeatable, scalable evaluation using computed scores.

Text Generation:

BLEU: N-gram overlap (translation)
ROUGE: Recall-oriented (summarization)
METEOR: Semantic similarity
BERTScore: Embedding-based similarity
Perplexity: Language model confidence

Classification:

Accuracy: Percentage correct
Precision/Recall/F1: Class-specific performance
Confusion Matrix: Error patterns
AUC-ROC: Ranking quality

Retrieval (RAG):

MRR: Mean Reciprocal Rank
NDCG: Normalized Discounted Cumulative Gain
Precision@K: Relevant in top K
Recall@K: Coverage in top K

2. Human Evaluation

Manual assessment for quality aspects difficult to automate.

Dimensions:

Accuracy: Factual correctness
Coherence: Logical flow
Relevance: Answers the question
Fluency: Natural language quality
Safety: No harmful content
Helpfulness: Useful to the user

3. LLM-as-Judge

Use stronger LLMs to evaluate weaker model outputs.

Approaches:

Pointwise: Score individual responses
Pairwise: Compare two responses
Reference-based: Compare to gold standard
Reference-free: Judge without ground truth

Quick Start

from llm_eval import EvaluationSuite, Metric

# Define evaluation suite
suite = EvaluationSuite([
    Metric.accuracy(),
    Metric.bleu(),
    Metric.bertscore(),
    Metric.custom(name="groundedness", fn=check_groundedness)
])

# Prepare test cases
test_cases = [
    {
        "input": "What is the capital of France?",
        "expected": "Paris",
        "context": "France is a country in Europe. Paris is its capital."
    },
    # ... more test cases
]

# Run evaluation
results = suite.evaluate(
    model=your_model,
    test_cases=test_cases
)

print(f"Overall Accuracy: {results.metrics['accuracy']}")
print(f"BLEU Score: {results.metrics['bleu']}")

Automated Metrics Implementation

BLEU Score

from nltk.translate.bleu_score import sentence_bleu, SmoothingFunction

def calculate_bleu(reference, hypothesis):
    """Calculate BLEU score between reference and hypothesis."""
    smoothie = SmoothingFunction().method4

    return sentence_bleu(
        [reference.split()],
        hypothesis.split(),
        smoothing_function=smoothie
    )

# Usage
bleu = calculate_bleu(
    reference="The cat sat on the mat",
    hypothesis="A cat is sitting on the mat"
)

ROUGE Score

from rouge_score import rouge_scorer

def calculate_rouge(reference, hypothesis):
    """Calculate ROUGE scores."""
    scorer = rouge_scorer.RougeScorer(['rouge1', 'rouge2', 'rougeL'], use_stemmer=True)
    scores = scorer.score(reference, hypothesis)

    return {
        'rouge1': scores['rouge1'].fmeasure,
        'rouge2': scores['rouge2'].fmeasure,
        'rougeL': scores['rougeL'].fmeasure
    }

BERTScore

from bert_score import score

def calculate_bertscore(references, hypotheses):
    """Calculate BERTScore using pre-trained BERT."""
    P, R, F1 = score(
        hypotheses,
        references,
        lang='en',
        model_type='microsoft/deberta-xlarge-mnli'
    )

    return {
        'precision': P.mean().item(),
        'recall': R.mean().item(),
        'f1': F1.mean().item()
    }

Custom Metrics

def calculate_groundedness(response, context):
    """Check if response is grounded in provided context."""
    # Use NLI model to check entailment
    from transformers import pipeline

    nli = pipeline("text-classification", model="microsoft/deberta-large-mnli")

    result = nli(f"{context} [SEP] {response}")[0]

    # Return confidence that response is entailed by context
    return result['score'] if result['label'] == 'ENTAILMENT' else 0.0

def calculate_toxicity(text):
    """Measure toxicity in generated text."""
    from detoxify import Detoxify

    results = Detoxify('original').predict(text)
    return max(results.values())  # Return highest toxicity score

def calculate_factuality(claim, knowledge_base):
    """Verify factual claims against knowledge base."""
    # Implementation depends on your knowledge base
    # Could use retrieval + NLI, or fact-checking API
    pass

LLM-as-Judge Patterns

Single Output Evaluation

def llm_judge_quality(response, question):
    """Use GPT-5 to judge response quality."""
    prompt = f"""Rate the following response on a scale of 1-10 for:
1. Accuracy (factually correct)
2. Helpfulness (answers the question)
3. Clarity (well-written and understandable)

Question: {question}
Response: {response}

Provide ratings in JSON format:
{{
  "accuracy": <1-10>,
  "helpfulness": <1-10>,
  "clarity": <1-10>,
  "reasoning": "<brief explanation>"
}}
"""

    result = openai.ChatCompletion.create(
        model="gpt-5",
        messages=[{"role": "user", "content": prompt}],
        temperature=0
    )

    return json.loads(result.choices[0].message.content)

Pairwise Comparison

def compare_responses(question, response_a, response_b):
    """Compare two responses using LLM judge."""
    prompt = f"""Compare these two responses to the question and determine which is better.

Question: {question}

Response A: {response_a}

Response B: {response_b}

Which response is better and why? Consider accuracy, helpfulness, and clarity.

Answer with JSON:
{{
  "winner": "A" or "B" or "tie",
  "reasoning": "<explanation>",
  "confidence": <1-10>
}}
"""

    result = openai.ChatCompletion.create(
        model="gpt-5",
        messages=[{"role": "user", "content": prompt}],
        temperature=0
    )

    return json.loads(result.choices[0].message.content)

Human Evaluation Frameworks

Annotation Guidelines

class AnnotationTask:
    """Structure for human annotation task."""

    def __init__(self, response, question, context=None):
        self.response = response
        self.question = question
        self.context = context

    def get_annotation_form(self):
        return {
            "question": self.question,
            "context": self.context,
            "response": self.response,
            "ratings": {
                "accuracy": {
                    "scale": "1-5",
                    "description": "Is the response factually correct?"
                },
                "relevance": {
                    "scale": "1-5",
                    "description": "Does it answer the question?"
                },
                "coherence": {
                    "scale": "1-5",
                    "description": "Is it logically consistent?"
                }
            },
            "issues": {
                "factual_error": False,
                "hallucination": False,
                "off_topic": False,
                "unsafe_content": False
            },
            "feedback": ""
        }

Inter-Rater Agreement

from sklearn.metrics import cohen_kappa_score

def calculate_agreement(rater1_scores, rater2_scores):
    """Calculate inter-rater agreement."""
    kappa = cohen_kappa_score(rater1_scores, rater2_scores)

    interpretation = {
        kappa < 0: "Poor",
        kappa < 0.2: "Slight",
        kappa < 0.4: "Fair",
        kappa < 0.6: "Moderate",
        kappa < 0.8: "Substantial",
        kappa <= 1.0: "Almost Perfect"
    }

    return {
        "kappa": kappa,
        "interpretation": interpretation[True]
    }

A/B Testing

Statistical Testing Framework

from scipy import stats
import numpy as np

class ABTest:
    def __init__(self, variant_a_name="A", variant_b_name="B"):
        self.variant_a = {"name": variant_a_name, "scores": []}
        self.variant_b = {"name": variant_b_name, "scores": []}

    def add_result(self, variant, score):
        """Add evaluation result for a variant."""
        if variant == "A":
            self.variant_a["scores"].append(score)
        else:
            self.variant_b["scores"].append(score)

    def analyze(self, alpha=0.05):
        """Perform statistical analysis."""
        a_scores = self.variant_a["scores"]
        b_scores = self.variant_b["scores"]

        # T-test
        t_stat, p_value = stats.ttest_ind(a_scores, b_scores)

        # Effect size (Cohen's d)
        pooled_std = np.sqrt((np.std(a_scores)**2 + np.std(b_scores)**2) / 2)
        cohens_d = (np.mean(b_scores) - np.mean(a_scores)) / pooled_std

        return {
            "variant_a_mean": np.mean(a_scores),
            "variant_b_mean": np.mean(b_scores),
            "difference": np.mean(b_scores) - np.mean(a_scores),
            "relative_improvement": (np.mean(b_scores) - np.mean(a_scores)) / np.mean(a_scores),
            "p_value": p_value,
            "statistically_significant": p_value < alpha,
            "cohens_d": cohens_d,
            "effect_size": self.interpret_cohens_d(cohens_d),
            "winner": "B" if np.mean(b_scores) > np.mean(a_scores) else "A"
        }

    @staticmethod
    def interpret_cohens_d(d):
        """Interpret Cohen's d effect size."""
        abs_d = abs(d)
        if abs_d < 0.2:
            return "negligible"
        elif abs_d < 0.5:
            return "small"
        elif abs_d < 0.8:
            return "medium"
        else:
            return "large"

Regression Testing

Regression Detection

class RegressionDetector:
    def __init__(self, baseline_results, threshold=0.05):
        self.baseline = baseline_results
        self.threshold = threshold

    def check_for_regression(self, new_results):
        """Detect if new results show regression."""
        regressions = []

        for metric in self.baseline.keys():
            baseline_score = self.baseline[metric]
            new_score = new_results.get(metric)

            if new_score is None:
                continue

            # Calculate relative change
            relative_change = (new_score - baseline_score) / baseline_score

            # Flag if significant decrease
            if relative_change < -self.threshold:
                regressions.append({
                    "metric": metric,
                    "baseline": baseline_score,
                    "current": new_score,
                    "change": relative_change
                })

        return {
            "has_regression": len(regressions) > 0,
            "regressions": regressions
        }

Benchmarking

Running Benchmarks

class BenchmarkRunner:
    def __init__(self, benchmark_dataset):
        self.dataset = benchmark_dataset

    def run_benchmark(self, model, metrics):
        """Run model on benchmark and calculate metrics."""
        results = {metric.name: [] for metric in metrics}

        for example in self.dataset:
            # Generate prediction
            prediction = model.predict(example["input"])

            # Calculate each metric
            for metric in metrics:
                score = metric.calculate(
                    prediction=prediction,
                    reference=example["reference"],
                    context=example.get("context")
                )
                results[metric.name].append(score)

        # Aggregate results
        return {
            metric: {
                "mean": np.mean(scores),
                "std": np.std(scores),
                "min": min(scores),
                "max": max(scores)
            }
            for metric, scores in results.items()
        }

Resources

references/metrics.md: Comprehensive metric guide
references/human-evaluation.md: Annotation best practices
references/benchmarking.md: Standard benchmarks
references/a-b-testing.md: Statistical testing guide
references/regression-testing.md: CI/CD integration
assets/evaluation-framework.py: Complete evaluation harness
assets/benchmark-dataset.jsonl: Example datasets
scripts/evaluate-model.py: Automated evaluation runner

Best Practices

Multiple Metrics: Use diverse metrics for comprehensive view
Representative Data: Test on real-world, diverse examples
Baselines: Always compare against baseline performance
Statistical Rigor: Use proper statistical tests for comparisons
Continuous Evaluation: Integrate into CI/CD pipeline
Human Validation: Combine automated metrics with human judgment
Error Analysis: Investigate failures to understand weaknesses
Version Control: Track evaluation results over time

Common Pitfalls

Single Metric Obsession: Optimizing for one metric at the expense of others
Small Sample Size: Drawing conclusions from too few examples
Data Contamination: Testing on training data
Ignoring Variance: Not accounting for statistical uncertainty
Metric Mismatch: Using metrics not aligned with business goals

llm-evaluation

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