Senior Data Scientist, ML

About the role... We're looking for a Senior Data Scientist focused on ML to own the analytics and experimentation layer of our recommendation and discovery systems in Warsaw or New York City. You'll partner directly with our Head of Product and Staff ML Engineer to shape how millions of people find inspiration on Cosmos. This is a high-ownership role at an early-stage company. You'll design the frameworks we use to understand content relevance and user engagement, run experiments that directly influence ranking and personalization, and help us build a recommendation ecosystem that balances discovery, creator distribution, and long-term user value. If you've worked on recommendation systems at scale and want to apply that expertise somewhere you can see the full picture and move fast, this is the role. What you'll do: Design and analyze experiments across our recommendation surfaces, including multi-arm A/B tests and ecosystem-level tradeoff analysis Develop frameworks to explain and improve content ranking and relevance—understanding why certain content gets promoted (or doesn't) for specific users Partner with ML Engineering on algorithm development, feature analysis, and model evaluation Quantify opportunities across our discovery ecosystem (boards, collections, trending content, creator distribution) and translate insights into roadmap priorities Define and monitor north star metrics, building a deep understanding of user behavior and long-term engagement patterns Influence product strategy with data—you'll have a seat at the table, not just a ticket queue What we're looking for: 7+ years of experience in data science, with significant time spent on recommendation systems, ranking, or personalization Strong experimentation chops—you've designed and analyzed complex A/B tests, understand causal inference, and can navigate ecosystem-level tradeoffs Proficiency in SQL and Python, with experience working on web-scale data Track record of influencing product strategy and shipping improvements based on your analysis Experience partnering closely with ML engineers on algorithm development and evaluation Ability to communicate clearly to technical and non-technical audiences—you tell stories with data, not just present charts Comfort with ambiguity and excitement about building foundational data practices at an early-stage company Nice to have: Experience at a visual discovery, social, or content platform (Pinterest, Instagram, TikTok, etc.) Background in balancing organic and paid/promoted content distribution Experience with contextual bandits, reinforcement learning, or online learning systems Benefits & perks: Premium health, dental, and vision 20 days PTO, plus company holidays, plus a 2-week winter break Top-spec MacBook Pro, Apple Studio Display Monthly stipend for software and tools Monthly team events Fully covered commute in NYC Daily lunch & dinner stipend at the office Optional Superpower membership Join our Team We'd love to hear from you.