We will conduct an interactive tutorial describing how to evaluate the impact of language design decisions on people, focusing on objective and data-driven methods. This includes a wide variety of quantitative and qualitative empirical techniques, strategies for conducting experiments or randomized controlled trials, and case studies of what can/cannot be tackled using such methods. We will also discuss the evidence for or against competing design choices, showing what current beliefs in language design are confirmed, plausible, busted, or which have not been tested. Finally, participants will practice designing experimental protocols, obtaining expert feedback on the alternatives.