Not before time, the Programme for International Student Assessment (Pisa) is facing increasing criticism ("Pisa protestations ring hollow", Letters, 27 September). Its leadership is twisting and turning as it seeks to combat its critics. When will Pisa admit that students' understanding, whether in developed or developing countries, cannot possibly be "measured" by any of its tests, however sophisticated its statistical analyses and however elaborate its rationalisations? There is no substitute for judgement of students' learning by teachers who work closely with them. Yet that judgement is fallible. That may be uncomfortable to acknowledge but that doesn't make it less true. We live and work in an imperfect, imprecise and untidy world.
Colin Richards, Spark Bridge, Cumbria.