Why you care: Before you can run any experiments, you must have instrumentation in place to log what is happening to the users and the system (e.g., website, application). Moreover, every business should have a baseline understanding of how the system is performing and how users interact with it, which requires instrumentation. When running experiments, having rich data about what users saw, their interactions (e.g., clicks, hovers, and time-to-click), and system performance (e.g., latencies) is critical.