Secondary school performance in Wales revealed
Figures showing whether secondary schools in Wales exceed expectations or perform worse than expected have been obtained for the first time.
The Welsh Government data, requested by BBC Wales, shows 19% of schools performed below expectations based on last year's GCSE examinations.
Another 22% did better than predicted while 60% of schools performed to the expected level.
The Welsh Government said it did not believe in "simplistic league tables".
The information is one of a number of performance measures used by schools, local authorities and the Welsh Government but never before made public.
The data is considered by experts to be among the most reliable indicators of school performance because it shows the extent to which a school has contributed to a pupil's progress.
It was obtained under the Freedom of Information Act by BBC Wales and is the most comprehensive school performance data to be published since league tables were abolished in 2001.
The measure is known as value added data.
Schools are scored based on whether each pupil's GCSE results are better or worse than what was predicted of them at the age of eleven.
The measure singles out schools which enable pupils to exceed expectations, whether, for example, it is an E grade student who achieves a C, or a B grade student who achieves an A*.
However, it also raises questions about schools where pupils receive significantly lower grades than expected.
Councils in Wales where school performance was rated lowest said strategies were already in place to improve results in core subjects.
But some raised questions about whether the figures offered the most accurate picture.
Pembrokeshire council said: "The authority concurs with the view of the minister for education and skills that simplistic league tables can be divisive and misleading and do not, in themselves, promote improvement."
The data released to the BBC by the Welsh Government shows that Cynffig Comprehensive in Bridgend was the school that scored highest in 2010. Denbigh High School in Denbighshire recorded the lowest score.
The measure is considered robust because it takes into account factors outside a school's control which are known to affect exam performance, such as poverty levels and pupils' ethnicity, gender and exact age.
As such the scores do not discriminate against schools in more deprived areas.
Such schools usually perform less well in conventional school league tables, which are based primarily on the number of pupils achieving five A* to C grades at GCSE and tend to favour schools in the wealthiest parts of the country.
Value added scores, however, are calculated by estimating what a pupil is expected to achieve at GCSE, including the core subjects of English or Welsh and mathematics, when they are aged 11.
This is compared with their actual performance aged 16 and a score is produced after being put through a formula to account for factors such as poverty.
If a school has a positive score pupils have exceeded expectations, a negative score indicates that pupils at the school have performed below expectations.
The data identifies whether a school's result is significantly positive, significantly negative or neutral, which means that pupils have mostly performed according to expectations.
A negative score would be a cause for concern because pupils would have achieved GCSE results significantly below what was expected of them when they entered the school.
The data also reveals which local authorities are considered to be doing well on this measure.
In Flintshire more than half the schools received significantly positive scores, but in Denbighshire, Merthyr Tydfil, Pembrokeshire, Monmouthshire or Ceredigion not a single school was found to significantly exceed expectations.
The scores show considerable variation and raise questions about the effectiveness of school improvement in certain areas.
For instance, Swansea received a significantly positive score while Cardiff was among the group of worst performing authorities.
A Welsh Government spokesperson said: "The minister has made it clear that he wants to raise school performance and standards across the board and has outlined exactly how we are going to achieve that.
"Our policy on not publishing league tables is well known. We do not believe that simplistic league tables have a role in the improvement agenda for Wales and that is why we scrapped them.
"This is an extremely simplistic league table ranking schools according to a single piece of data from one school year.
"Our value added model compares an individual school's results to the overall results for Wales, so this means there will always be around half positive and half negative using this approach. Expecting all schools to have positive value added is a bit like expecting everyone to be above average.
"To get a genuine view of a school's performance you need to take account of a range of data, look at progress and context - what the BBC have done simply does not reflect what any school improvement professional or teacher would recognise as an informed and rounded view of school performance. "
The results comes as Education Minister Leighton Andrews is set to push on with the school changes agenda set out before the assembly elections.
Following poor performance in international assessments the minister accused schools of "complacency in the classroom" and spoke of "systemic failure" in the school system.
In a speech in February, the minister called on local authorities to do more to address school performance and announced proposals to introduce a system of grading schools.
BBC Wales will reveal more school performances obtained under the Freedom of Information Act next week.