Be sure to watch Andreas Schleicher's TED talk (referenced by Sjoberg) to hear PISA's rationale. This video is widely cited by PISA haters, but I've never seen Schleicher's talk cited in the same context. Note too that the German Institute for International Educational Research found that 85% of the between-country variation in PISA Mathematics Literacy could be explained by TIMSS, and vice versa. They reported a similar finding for science. So Sjoberg's criticisms of curriculum not being included in PISA doesn't seem to matter when it comes down to testing whether students can actually think independently in the end.