Making an inference involves drawing a general conclusion from specific observations. People do this every day. Upon arising in the morning, one observes that the sun is shining and that the day will be nice. The news reports the arrest of a military veteran for child abuse, and a listener infers that military veterans have special adjustment problems. Statistical inference is a way of formalizing the process of drawing general conclusions from limited information. It is a way of stating the degree of confidence one has in making an inference by using probability theory. Statistically based research allows people to move beyond speculation.

Suppose a sociologist interviews two husbands. Josh, whose wife is employed, does 50 percent of the household chores; Frank, whose wife does not work for pay, does 10 percent. Should the sociologist infer that husbands do more housework when their wives are employed? No. This...