At the moment, the Department for Education is considering changes to the league tables and the exam system. This seems an opportune moment to make a simple point about qualification-awarding and accountability: English school examinations are subject to measurement error in a really big way.
Here is a simple thought experiment to flesh it out. Imagine a class of 100 students. Let us specify that each one has a “true” ability that means that one pupil should get one point, one pupil should get two, one should get three and so on – up to 100 marks. Now, let’s award this class one of 10 grades: 90+ gets you an A, 80+ a B and so on.
Let us assume that the tests are perfect. If that were the case, you would get ten individuals in each grade. Easy enough. But what happens if we start introducing errors into the test? We can do that with a set of exotically named (but very simple) “Monte Carlo” estimates, which I calculated using this simple spreadsheet. Read more