---

name: "algo-rec-hybrid" description: "Design hybrid recommendation systems combining multiple strategies for improved accuracy. Use this skill when the user needs to overcome single-method limitations, combine collaborative and content-based filtering, or build a production recommendation pipeline — even if they say 'combine recommendation approaches', 'best recommendation architecture', or 'cold start plus personalization'." metadata: category: "WP-36 推薦系統" tags: ["recommendation", "hybrid", "ensemble", "system-design"]

Hybrid Recommendation System

Overview

Hybrid recommendation combines multiple strategies (CF, content-based, knowledge-based) to overcome individual method limitations. Common architectures: weighted, switching, cascade, feature augmentation, and meta-level. Complexity varies by architecture.

When to Use

Trigger conditions:

• Building a production recommendation system that must handle cold start AND personalization
• Single methods have known weaknesses for your use case
• Need to balance accuracy, diversity, and coverage

When NOT to use:

• When you have a single clean data source (start with the matching single method first)
• When system simplicity is more important than marginal accuracy gains

Algorithm

IRON LAW: Hybrid Adds Value ONLY With Complementary Strengths
Combining two systems with the SAME weakness amplifies the weakness.
CF fails on cold start + content-based fails on cold start = hybrid
STILL fails on cold start. Choose components that cover each other's gaps.

Phase 1: Input Validation

Identify available data: interaction history (for CF), item features (for content-based), contextual signals (time, device, location). Map data to method capabilities. Gate: At least two complementary data sources available.

Phase 2: Core Algorithm

Weighted hybrid: Score = α × CF_score + β × CB_score. Tune weights via cross-validation.

Switching hybrid: Use CF when sufficient data exists; switch to content-based for cold start items/users.

Cascade hybrid: First stage filters (e.g., content-based), second stage ranks (e.g., CF) within filtered set.

Feature augmentation: Use one method's output as input features for another (e.g., CF embeddings as content features).

Phase 3: Verification

A/B test hybrid vs individual components. Measure: accuracy (NDCG, precision@K), coverage (% of catalog recommended), diversity (intra-list diversity). Gate: Hybrid outperforms best individual component on primary metric.

Phase 4: Output

Return recommendations with source attribution for explainability.

Output Format

{
  "recommendations": [{"item_id": "789", "score": 0.91, "sources": {"cf": 0.85, "content": 0.95}, "method": "weighted"}],
  "metadata": {"architecture": "weighted", "weights": {"cf": 0.6, "content": 0.4}, "coverage": 0.78}
}

Examples

Sample I/O

Input: New user with 2 interactions + rich item feature catalog Expected: Switching hybrid: content-based recommendations (insufficient CF data), transitioning to CF as interactions accumulate

Edge Cases

Input	Expected	Why
Completely new user + new item	Fall back to popularity	No data for either method
Methods disagree strongly	Depends on architecture	Weighted averages; cascade defers to second stage
One component returns empty	Other component takes over	Graceful degradation

Gotchas

• Complexity cost: Each added component increases latency, maintenance, and debugging difficulty. Start simple, add complexity only when justified by metrics.
• Weight tuning: Static weights degrade over time. Retune periodically or use learned weights (e.g., a meta-model that predicts which component performs best per context).
• Evaluation is harder: You must evaluate the hybrid AND each component individually to understand contribution and detect regressions.
• Feature leakage: In feature augmentation, ensure the augmenting model's predictions don't leak test-set information during training.
• Diminishing returns: Going from one method to two gives the biggest lift. Adding a third rarely justifies the complexity.

References

• For architecture selection decision guide, see references/architecture-selection.md
• For A/B testing recommendation systems, see references/ab-testing-recs.md