One good way to illustrate that is by counterexamples. In graduate school, I heard a story from my fellow students, out on a consultation, about a client who was giving IQ tests to hire people to load heavy sacks into boxcars. They had convinced their client to abandon the use of that test, and instead devised one of their own. Prospective applicants were taken to a room where a number of sacks had been placed near a shelf. The applicants then had to put the sacks on the shelf.
Validity is local because a test and a task correlate.
Loading the sacks in the test situation will correlate with loading the sacks in the field. Similarly, the items on tests like the SAT correlate with the tasks required in college. So, it is more valid to give the SAT to high school seniors applying to college, but not valid to give it to high school seniors learning a trade. In the latter case, other, more valid tests could be devised. Wrote Introduction to Comparative Psychology Developed Morgan's Canon, a modern version of Occam's Razor Animal psychology was a new and original kind of psychology compared to voluntarism and structuralism URLs Talk.