TL;DR
Accuracy can be misleading with imbalanced data. Use precision when false positives are costly, recall when false negatives are costly, and F1 when you need balance. For retrieval, MRR measures first-result quality, NDCG measures ranking quality.
Visual Overview
┌───────────────────────────────────────────────────────────┐ │ │ │ Predicted │ │ Pos Neg │ │ ┌────────┬────────┐ │ │ Actual Pos │ TP │ FN │ │ │ ├────────┼────────┤ │ │ Neg │ FP │ TN │ │ │ └────────┴────────┘ │ │ │ │ TP = True Positive (correct positive) │ │ FP = False Positive (false alarm) │ │ FN = False Negative (missed) │ │ TN = True Negative (correct negative) │ │ │ └───────────────────────────────────────────────────────────┘
Classification Metrics
Precision
┌───────────────────────────────────────────────────────────┐ │ │ │ Precision = TP / (TP + FP) │ │ │ │ "Of everything I predicted positive, how many were │ │ actually positive?" │ │ │ │ High precision = few false alarms │ │ Optimize when: false positives are costly │ │ (spam filter, fraud detection) │ │ │ └───────────────────────────────────────────────────────────┘
Recall
┌───────────────────────────────────────────────────────────┐ │ │ │ Recall = TP / (TP + FN) │ │ │ │ "Of everything that was actually positive, how many │ │ did I find?" │ │ │ │ High recall = few misses │ │ Optimize when: false negatives are costly │ │ (disease detection, security threats) │ │ │ └───────────────────────────────────────────────────────────┘
F1 Score
┌───────────────────────────────────────────────────────────┐ │ │ │ F1 = 2 x (Precision x Recall) / (Precision + Recall) │ │ │ │ Harmonic mean of precision and recall. │ │ Use when: you need single number, classes are imbalanced│ │ │ │ Note: Harmonic mean punishes extreme imbalance harder │ │ than arithmetic mean. │ │ │ └───────────────────────────────────────────────────────────┘
Precision-Recall Tradeoff
TRADEOFF ┌───────────────────────────────────────────────────────────┐ │ │ │ High Threshold Low Threshold │ │ (conservative) (aggressive) │ │ │ │ │ │ ▼ ▼ │ │ ┌──────────────┐ ┌──────────────┐ │ │ P │ HIGH │ │ LOW │ │ │ └──────────────┘ └──────────────┘ │ │ ┌──────────────┐ ┌──────────────┐ │ │ R │ LOW │ │ HIGH │ │ │ └──────────────┘ └──────────────┘ │ │ │ │ Moving threshold trades one for the other. │ │ │ └───────────────────────────────────────────────────────────┘
Threshold Tuning
Default threshold is 0.5. This is arbitrary.
┌───────────────────────────────────────────────────────────┐ │ │ │ The right threshold depends on your cost function: │ │ │ │ FN 10x worse than FP? → Lower threshold │ │ FP 10x worse than FN? → Higher threshold │ │ Equal cost? → Optimize F1 │ │ │ └───────────────────────────────────────────────────────────┘
Common threshold strategies:
| Strategy | When to use |
|---|---|
| Maximize F1 | Balanced importance |
| Fixed recall (e.g., 95%) | Can’t miss positives (medical) |
| Fixed precision (e.g., 95%) | Can’t have false alarms (spam) |
| Cost-weighted | Know exact cost of FP vs FN |
Class Imbalance
THE PROBLEM ┌───────────────────────────────────────────────────────────┐ │ │ │ Dataset: 99% negative, 1% positive (fraud detection) │ │ │ │ Model predicts "negative" for everything: │ │ Accuracy = 99% ← Looks great! │ │ Recall = 0% ← Completely useless │ │ │ │ Accuracy is misleading with imbalanced classes. │ │ │ └───────────────────────────────────────────────────────────┘
Solutions:
1. USE DIFFERENT METRICS ┌───────────────────────────────────────────────────────────┐ │ │ │ Bad: Accuracy │ │ Good: F1, Precision, Recall, PR-AUC │ │ │ │ F1 on "always negative" model = 0 (reveals the problem) │ │ │ └───────────────────────────────────────────────────────────┘ 2. PR-AUC OVER ROC-AUC ┌───────────────────────────────────────────────────────────┐ │ │ │ ROC-AUC can look good with imbalanced data (high TNR) │ │ PR-AUC focuses on positive class performance │ │ │ │ Severe imbalance? PR-AUC is the metric. │ │ │ └───────────────────────────────────────────────────────────┘ 3. MACRO VS MICRO AVERAGING ┌───────────────────────────────────────────────────────────┐ │ │ │ Multi-class with imbalance: │ │ │ │ Micro F1: Aggregate TP, FP, FN across all classes │ │ → Dominated by majority class │ │ │ │ Macro F1: Compute F1 per class, then average │ │ → Each class weighted equally │ │ │ │ Imbalanced? Use Macro F1 to ensure minority classes │ │ matter. │ │ │ └───────────────────────────────────────────────────────────┘
Retrieval Metrics
Precision@K and Recall@K
┌───────────────────────────────────────────────────────────┐ │ │ │ P@K = (relevant docs in top K) / K │ │ "Of the K results I returned, how many were relevant?" │ │ │ │ R@K = (relevant docs in top K) / (total relevant docs) │ │ "Of all relevant docs, how many appear in my top K?" │ │ │ └───────────────────────────────────────────────────────────┘
Mean Reciprocal Rank (MRR)
MRR ┌───────────────────────────────────────────────────────────┐ │ │ │ MRR = (1/|Q|) x SUM(1 / rank_i) │ │ │ │ Where rank_i = position of first relevant result │ │ │ │ Example: │ │ Query 1: first relevant at position 3 → 1/3 │ │ Query 2: first relevant at position 1 → 1/1 │ │ Query 3: first relevant at position 2 → 1/2 │ │ │ │ MRR = (1/3 + 1 + 1/2) / 3 = 0.61 │ │ │ │ Use when: you care most about the first relevant result │ │ │ └───────────────────────────────────────────────────────────┘
NDCG (Normalized Discounted Cumulative Gain)
NDCG ┌───────────────────────────────────────────────────────────┐ │ │ │ DCG@K = SUM(relevance_i / log2(i + 1)) │ │ │ │ NDCG@K = DCG@K / IDCG@K (normalized by ideal ranking) │ │ │ │ Accounts for: │ │ • Graded relevance (not just binary) │ │ • Position discount (top results matter more) │ │ │ │ Use when: relevance has degrees, ranking order matters │ │ │ └───────────────────────────────────────────────────────────┘
When to Use What
| Scenario | Metric | Why |
|---|---|---|
| Binary classification, balanced | Accuracy | Simple, interpretable |
| Binary classification, imbalanced | F1 or PR-AUC | Accuracy misleading |
| Multi-class, balanced | Accuracy or Micro F1 | Simple aggregate |
| Multi-class, imbalanced | Macro F1 | Each class matters equally |
| Retrieval, one right answer | MRR | First result matters |
| Retrieval, multiple relevant | NDCG | Ranking quality |
| RAG retrieval component | Recall@K | Did we retrieve the context? |
| Medical/safety-critical | Recall | Can’t miss positives |
| Spam/fraud filtering | Precision | Can’t have false alarms |
Debugging with Metrics
HIGH ACCURACY BUT POOR REAL-WORLD PERFORMANCE ┌───────────────────────────────────────────────────────────┐ │ │ │ Symptoms: │ │ • Model accuracy is 95% │ │ • Users complain it doesn't work │ │ │ │ Causes: │ │ • Class imbalance (predicting majority class) │ │ • Test set doesn't match production distribution │ │ • Wrong metric for actual goal │ │ │ │ Debug steps: │ │ 1. Check class balance in test set │ │ 2. Compute per-class metrics (confusion matrix) │ │ 3. Compute F1, not just accuracy │ │ 4. Sample production data and evaluate │ │ │ └───────────────────────────────────────────────────────────┘ PRECISION AND RECALL BOTH LOW ┌───────────────────────────────────────────────────────────┐ │ │ │ Symptoms: │ │ • P = 0.4, R = 0.3 (both bad) │ │ • Model seems random │ │ │ │ Causes: │ │ • Model not learning (training issue) │ │ • Features not predictive │ │ • Data quality issues │ │ │ │ Debug steps: │ │ 1. Check training loss - is it decreasing? │ │ 2. Overfit to small dataset first │ │ 3. Check feature importance │ │ 4. Examine misclassified examples manually │ │ │ └───────────────────────────────────────────────────────────┘
When This Matters
| Situation | What to know |
|---|---|
| Evaluating any classifier | Check class balance first |
| Imbalanced data | Use F1 or PR-AUC, not accuracy |
| Multi-class imbalance | Use Macro F1 |
| Setting classification threshold | Tune based on cost of FP vs FN |
| Evaluating retrieval | MRR for single answer, NDCG for ranking |
| RAG system evaluation | Recall@K for retriever |
| Model seems good but users complain | Metric doesn’t match user goal |
Production signal
Why this concept matters
Interview 75% of ML interviews
Production Every model evaluation
Performance Choosing right metric for task