An international test of adults’ “basic skills” shows that an increasing number of Americans are struggling to do moderately complex tasks that involve reading and math. Unlike tests given to students, these tests attempt to evaluate real-life skills, like whether a person can understand what a list of rules prohibits or calculate how much wallpaper is needed to cover a room—or, at more advanced levels, draw conclusions based on multiple sources or evaluate statistical claims.Known as the PIAAC,...