The use of test scores as evidence to support the claims made for them requires an understanding of causal inference. We provide a careful discussion of the modern theory of causal inferences with numerous evocative illustrations, including an admissions policy at the University of Nebraska, the 1854 London cholera epidemic, and the 1960s decline in SAT scores. We show how evidence drawn from test scores is comparable to credible evidence of other widely accepted sources. Rubin’s model for causal inference is explained and the importance of manipulation, random assignment, potential outcomes, and a control group is emphasized. The Tennessee Class Size Experiment of the 1980s is one of the best examples of how to measure the effects of a cause. Finally, we show how the size of the causal effect of fracking on earthquakes in Oklahoma can be established using an observational study by mirroring the structure of an experiment. Measuring the size of causal effects of testing and its alternatives requires data and control. Often, the data are kept hidden to avoid ruining the good with the truth.