Doug Turnbull

Co-founder of OpenSource Connections (OSC); currently independent (previously Shopify, then Reddit); co-author of Relevant Search (O’Reilly). Search relevance consultant and practitioner; author of the Softwaredoug blog.

Known For

• Relevant Search book (with John Berryman)
• Bayesian BM25 (BB25) — calibrated BM25 probabilities for principled hybrid fusion
• Practical Elasticsearch relevance tuning
• “What is Relevant?” — task-centered definition of relevance

Articles

• What is Presentation Bias in Search

• What AI Engineers Should Know about Search

• Bayesian BM25 is Cool

• What is a Relevant Search Result

• Practical BM25 - The Effect of k1 and b

• Semantic Search Without Embeddings

• Finding the Relevance Cutoff - When to Stop Showing Search Results

• Getting Started on Search Relevance for the Understaffed Search Team

• Principal Component Analysis - an embedding shrink-ray — PCA for embedding compression; measured recall vs. dimensions on MS MARCO

• Just brute force your embeddings — measured NumPy scan throughput on 384-dim vectors; the case against adopting a vector database at ~1m documents

• Updating a Vector Database Is No Simple Thing — tombstoning, segmented graphs, and vendor-by-vendor comparison of how HNSW indexes handle updates/deletes

Talks & Videos

• Haystack US 2022 - Bayesian Optimization of Relevance at Shopify — with Andy Toulis; Bayesian Optimization as a lightweight halfway point to Learning to Rank, with a concrete Shopify BM25 b/k1 retuning case study

Topics

• Tuning BM25 for E-commerce Search

Affiliations

• Independent (current)
• Reddit (former)
• Shopify (former)

Teaching

• Courses — teaches Cheat at Search Essentials (free intro), co-teaches AI-Powered Search with Trey Grainger, and teaches Build your own vector database (https://maven.com/softwaredoug/vectordb)

Key Concepts

• BM25
• Hybrid Search
• Learning to Rank
• Search Evaluation
• Judgment Lists
• Bayesian Optimization

Affiliations

• OpenSource Connections (OSC) — Doug co-founded it but has since left (later Shopify, now independent); also former org of Max Irwin (now at Bonsai)
• Can BM25 be a Probability — BB25 framework; calibrating BM25 as probability for principled hybrid fusion
• Metadata - The 3rd Kind of Retrieval — attribute-based retrieval as a third paradigm beyond lexical + embeddings
• Classic ML to Cope with Dumb LLM Judges — combining many dumb LLM judges via decision tree to improve precision
• Don’t Classify, Hallucinate — hypothetical classifications: let a cheap LLM invent a category, resolve it into the real taxonomy by embedding similarity; see Query Classification
• Agentic search models — the case for Purpose-Built Agentic Search Models for “the last 20%”; see topic Frontier of Search 2026

On Jev

Reads Jev through his standing argument for pairwise relevance judgment — that deciding one result beats another is easier than answering whether something is a 3/5 or a 4/5. LLMs had made that feasible but slow; a model tuned for yes/no decisions is what makes it compelling. See Reception of Jev.