Ministers are likely to face problems over the next few years assessing the degree to which schools are improving or getting worse because of changes in the way data from inspections are being collected.
The Office for Standards in Education, which is responsible for providing the Government with information on the state of the system, admitted this week that it has yet to resolve the way in which year-on-year comparisons can be made on the general state of schools.
An OFSTED spokesman said: "We may not be able to make direct statistical comparisons, but the statisticians are confident we will be able to identify trends."
The situation has arisen because OFSTED has changed the meaning of the middle point of the seven-point grading used by inspectors when making hundreds of judgments about schools. Future reports from Chris Woodhead, the chief inspector, using information collected from April will be based on a middle grade deemed to be positive.
The two previous years - 19956 and 19945 - will have been based on a middle grade deemed to be neutral and in "need of improvement".
The changes appear to have been made in order to ensure inspectors take a view on whether aspects of a school are satisfactory. However, the impact of the changes means the results cannot be compared with previous years.
The admission that there are question marks over future consistency and comparability of statistics comes as David Hart, leader of the largest headteachers' union, is urging Gillian Shephard, the Education and Employment Secretary, to institute an urgent inquiry into allegations made in The TES last week by a former senior inspector, Colin Richards, that OFSTED has manipulated and distorted data.
The National Association of Head Teachers maintains that the chief inspector, by changing the ground rules annually, is making nonsense of any attempt to arrive at valid conclusions. In his letter to Mrs Shephard, Mr Hart said: "The fact that a senior member of OFSTED is of the opinion that the statistics have been distorted and manipulated is very serious". Mr Hart is urging Mrs Shephard to inform Mr Woodhead that his report for this year will not be accepted until the allegations by Mr Richards have been investigated.
Mr Richards, former OFSTED primary specialist, alleges that the middle point on the scale was interpreted by Mr Woodhead as a negative grade in order to back up his statement that more than half of primary schools need to improve.
The accusation is denied by Mr Woodhead, who confirmed that the middle grade will be deemed to mean "in need of improvement" in next year's report. Reports that follow will be based on a positive meaning.
In a letter to The TES this week, he writes: "There can never be any question of manipulating the evidence. We must point to the strengths and weaknesses in current practice in a rigorous and dispassionate way".