Like all high-stakes testing, the Programme for International Student Assessment (Pisa) lends itself to manipulation, cheating or economies being taken with the truth - whether by states or by individual respondents (“Fresh Pisa claims shake trust in influential study”, 15 November). The findings reported from German and Canadian academics support those of us concerned not only about the quality of Pisa data but also the motivation of those providing it or using it for policy purposes. In high-stakes testing, as in war, the first casualty is the truth.

Colin Richards, Spark Bridge, Cumbria.