Hello world. My name is David J. Malan and I'm a professor of computer science at Harvard University. Today, I've been asked to explain algorithms in five levels of increasing difficulty ...

She says the A-level and IB grading algorithms fit her criteria for such WMDs, because they are important, opaque, and destructive. “They tick all the boxes,” she says.

This computer programmer phoned in to explain how the A-level algorithm works, and why it has led to widespread outrage.

Video: What on earth is an algorithm anyway? What are BTec Nationals and how do they work? Can I accept a degree place and sit the exam? And other questions Why did the A-level algorithm say no?