I’ve known this for a while. I first learned it when I was writing a test for my TEFL students. I had to figure out questions that were hard enough to demonstrate that they understood the material, but not so hard that they would be completely demoralized. I also had to make sure the questions were on the material the students legitimately should know, and not just on English language in general. When I started teaching HTML, I created a course, complete with an exam at the end, to demonstrate that students had learned something. The first iteration of the exam was fill in the blank. This made the test easier to write, but a lot harder to grade. So the second version was multiple guess. But multiple guess problems are hard to write because often the questions can be way too easy. For instance, how difficult would this be to guess the correct answer?