Designed for Use, 2nd Edition

Don’t Influence the Tester

Wilhelm von Osten was a German math teacher who lived around the early 1900s. As a hobby, he trained horses. You can probably see where this story is going: he tried to teach his horse how to do math. To everybody’s astonishment, the horse (aptly named Clever Hans) quickly learned to do a number of reasonably complex math calculations: additions, subtractions, multiplications, divisions, and date calculations. The horse was capable of understanding math questions asked in plain German, and it could also read math questions if they were written on a piece of paper.

Obviously, the horse couldn’t write down the results. Instead, it tapped the correct number with its hoof.

Psychologist Oskar Pfungst became suspicious and ...