Inference is about using facts one knows to learn about facts one does not know. Statistical inference is about examining a small piece of the world to learn about the entire world, along with evaluating the quality of the inference one reaches. Researchers call the “small part” a sample and the “world” a population.

People confront statistical inferences almost daily. When they open a newspaper, for example, they may find the results of a survey showing that 70 percent (95% CI ± 5) of American voters have confidence in the U.S. president. Or they may read about a scientific study indicating that a daily dose of aspirin helps 60 percent (95% CI ± 3) of Americans with heart disease (95% CI ± is explained below).

In neither ...