This document discusses A/B testing and its importance for lean startups. It provides examples of companies that conducted A/B tests, including Tropicana's redesign of its juice box packaging that resulted in a 20% drop in sales and cost $33 million. The presentation emphasizes that lean startups should be data-driven, listen to customers, and take a scientific approach using the scientific method of hypothesis testing. It also provides tips on designing effective A/B tests and names several A/B testing tools.