Controlled experimentation (A/B testing) allows companies to systematically study the effects of potential product changes or treatments by randomly assigning users to a control group or treatment group. The document discusses how controlled experiments can validate hypotheses with data, determine if a treatment has a causal effect, and provide examples of how A/B testing can be used for website variants, call-to-actions, and personalized recommendations. It also outlines best practices for running controlled experiments such as ensuring identical distributions between control and treatment groups and carefully monitoring each variant.