Good to read your much-needed article on the myth of Pisa - so beloved of the current dilettante inhabitant of the Department for Education. As I and a range of colleagues have been publicly attesting (for example, in the keynote lecture at the 2010 annual conference of the International Association for Educational Assessment), the instrument is not fit for purpose. Whatever Michael Davidson and his OECD buddies do to "weed out" (sic) the differential item functioning variations these are so huge across countries that they negate the Rasch methodology and produce only meaningless comparisons between those countries. Hopefully your article will return the debate to sound teaching and learning for student progress rather than a poorly norm-referenced league table.
Professor Bill Boyle, Chair of educational assessment, School of Education, University of Manchester.