In truth, people seem wedded to the idea that to get an average you must add numbers and divide (the mean). Back when I was teaching, down came the edict that when we wrote reports and entered exam marks, we should also write down the average mark in the class. The maths dept explained that the most sensible thing to do, if parents were to be given a proper idea of the class performance, was to write down the middle score (the median), together with the quarter and three-quarter scores. Needless to say, we were the only department that did: all the rest kept adding up the scores.
One of the biggest problems in maths was that when something was wrongly taught in primary school, such as an average means you have to add and divide, or a square is not a rectangle, it is almost impossible to shake it from students' minds. Perhaps that's what happened to the PACE authors. Perhaps they had bad teachers. Should Trish and I offer them some private tuition - Complete Beginners' Training?