The news from PISA

The latest results from the Program for International Student Assessment (PISA) are out and Ireland does not do too well. Sean Flynn has a fairly trenchant take on it. While I don't necessarily agree with everything he says, he makes some good points. The bottom line seems to be that we have slipped down the international league table significantly using the scores. PISA measures attainment of 15 year olds, usually in reading, science and mathematics. There have been several waves of data so far.
When the issue of "school league tables" comes up, many in the education sector are quick to oppose them on the basis that they don't provide a balanced comparison, not comparing like with like etc. Well if you believe that, you can't then use these "international league tables" to rank countries with different education systems, different cultures, different curricula and so on as you are certainly not comparing like with like either.
What I am unclear about is whether PISA only tells us where we are relative to others. That is, say attainment in Ireland has not changed in absolute terms but other countries have simply got better than us, then will we still move down the table? My guess is Yes but in reality its probably a bit of both going on: that is attainment may have fallen in Ireland and we may have been leapfroged by others also. My recollection is that PISA scores are normed to have an overall mean of 500 so it can't tell you about absolute changes but I may be wrong.