Schools must become more skilled in assessing their pupils’ performance in order to improve rates of academic achievement in Wales, the newly appointed chief inspector of education and training has said.

In his most candid interview since taking up the post, Dr Bill Maxwell said it was only by schools supplying clearer, more reliable and detailed statistics on their pupils that the Assembly government, local authorities and schools could successfully target weaker areas.

Dr Maxwell said he was no fan of “crude school league tables” which had been scrapped in Wales, but he believes what is needed are good comparisons between schools, with solid statistics.

New arrangements for pupil assessment in Wales at key stages 2 and 3 will come into force later in the spring. Sats have been slowly phased out in the age group since 2005 and were scrapped across Wales in January.

Teachers will have to assess all their pupils’ work internally at KS2, a move which some teaching unions say could add as much as 15 hours to the working week of staff away from the classroom.

The NASUWT has threatened to boycott the new arrangements and is holding talks with the Assembly government over fears of “nightmare” red tape.

But Dr Maxwell said improved internal assessment was essential for better standards. “Assessment is part of the core role of teaching and learning, and becoming skilled in this is important,” he said.

In Scotland, Dr Maxwell’s native country, a streamlined system of internal pupil assessment was made an important part of the 2005 document Assessment, Testing and Reporting. But unions see the proposals for Wales as not being streamlined enough, creating too much of a burden on schools already struggling to keep up.

Dr Maxwell, previously head of education and analytical services in the Scottish Executive Parliament, also indicated he was not in favour of lighter-touch inspections, which have support from most teaching unions.

He joins Estyn midway through a major review of school inspections in Wales, which will see changes from 2010.

The current system has been criticised by teaching unions, with the Association of School and College Leaders (ASCL) Cymru saying it is “past its sell-by date”.

Figures compiled by the union say that a three- to four-day inspection by a privately contracted team can cost between pound;10,000 and pound;15,000. Estyn says school inspection costs amounted to pound;3.4 million last year.

The union instead proposes that Estyn should base its verdicts - and decide if an inspection is necessary at all - on schools’ own self-assessment in a pre-inspection report, exam results and local authority reports.

Although Dr Maxwell said he supports moves towards better self-evaluation, and that it was his inspectorate’s job to make the best possible use of public money, he does not go as far as ASCL Cymru.

“I do not see a point at which we would be withdrawing from schools and just relying on self-evaluation,” Dr Maxwell said.

He claimed that just “looking at school paperwork, their policies and talking to senior management” would not give “a sound enough evidence base” for inspections. Estyn needed “corroboration” of what inspectors were told by going into schools and seeing teachers teach, he added.