We learned recently that, reflecting on past performance, Ofsted’s national direction of education, Sean Harford, considered that inspectors had been, in his words, “slow to pick up” on curriculum narrowing.

At the same time inspectors had, he thought, placed too great a reliance on data, and this had a particularly detrimental effect upon primary schools. Again, in Mr Harford’s words: “Because [performance measures] are so narrow in primary, people just focused in on those, and, as a consequence, much of the rest of the curriculum started to be denuded.”

It is refreshing that a leading inspector has the grace to admit that not everything has been rosy in the Ofsted garden and that, in important ways, Ofsted inspections have resulted in too many primary schools being forced on to a diet of Sats preparation that has, to employ a striking term used by Mr Harford, “denuded” the curriculum.

And that makes the prospect of Ofsted’s new inspection framework, unveiled upon our dazzled eyes recently, very interesting indeed.

Ofsted’s enormous challenge

For Ofsted is nothing if not ambitious.

Undaunted by its past failings where, by its own admission, it was asleep on the watch of curriculum narrowing, Ofsted has set itself an enormous challenge - not only to inspect the breadth and balance of a school’s curriculum, but also its quality.

So, inspectors must from this September judge whether schools provide a curriculum that is “ambitious”, designed to give pupils the “knowledge and cultural capital” to succeed in life, and that is “coherently planned and sequenced towards cumulatively sufficient knowledge and skills for future learning and employment”.

Note that nowhere in the inspection handbook are the important concepts of “ambition”, “cultural capital” and “skills” defined.

How, I wonder, are inspectors to make valid and reliable judgements on these complex concepts if they are not clearly defined with worked examples?

We are told that inspectors are receiving outstanding training that will transform their previous understandings and practice.

I hope that this is the case.

But I worry about the implications for inspectors who are, I believe, being asked to do the impossible with the inadequate, and the effect that this will have on schools, their staff and pupils who rely on Ofsted to give valid and reliable inspection judgements.

Education professionals are all too aware of the fact that Ofsted judgements are high stakes. High stakes for schools who may find themselves forcibly academised. High stakes for heads and teachers whose careers are, in too many cases, ended by a poor Ofsted judgement. And high stakes for those pupils who attend schools with poor Ofsted grades who, too often, see their school spinning into a cycle of decline.

Given these high stakes, I worry about even more inaccurate and invalid Ofsted judgements. Because the truth of the matter is that Ofsted is not going to get a bigger budget in order to make what are, undoubtedly, much more complex and nuanced inspection judgements.

For example, from September inspectors will be required to assess the quality of curriculum through a series of “deep dives”, which, according to the new Ofsted inspection handbook, are “a collection of deep, connected case studies of subjects, topics or aspects that can allow inspectors to form a valid and reliable view of the education on offer, provided that it is subject to further evidence-gathering to test systemic strengths and weaknesses of the curriculum”.

Tom Richmond, founder and director of EDSK, has analysed what a deep dive would mean in practice. Using calculations based on the instructions to inspectors in the new framework, he has calculated that a typical two-day inspection might involve up to nine hours of lesson observation in each inspection. (And this is based on inspectors spending 15 minutes in each lesson.)

Fair and reliable judgements?

Richmond also questions whether inspectors looking at the recommended sample of six exercise books from a year group of 100-200 pupils will produce fair and reliable judgements of whether pupils know more, remember more and can do more.

And there is a further, pressing problem with the new inspection framework - which is that generalist inspectors, who in most cases will have neither studied nor taught the subject they are inspecting beyond GCSE level, will be required to make highly complex judgements on teachers’ subject knowledge and on their pedagogy.

So, inspectors will be required to assess whether teachers identify pupil misconceptions accurately, and whether their teaching is designed to help learners to remember in the long term the content they have been taught.

Inspectors will be required to judge whether the resources chosen by the teacher clearly support the intent of a coherently planned curriculum, sequenced towards cumulatively sufficient knowledge and skills for future learning and employment.

I have asked this question before. And I will do so again. How will inspectors, without subject and/or age and phase qualifications and teaching experience, make these highly complex judgements?

I am afraid that I remain deeply worried that the new Ofsted framework is unmanageable and unworkable.

I fear that Ofsted’s inspection judgements will become even more unreliable and invalid than previously.

I conclude that tinkering at the edges will do no longer. England’s schools need, and deserve, an inspection system which is fair, valid, reliable and which serves to improve educational standards.

Ofsted is long past its sell-by date.

It is time for a new inspection deal for education professionals.

Mary Bousted is the joint-general secretary of the National Education Union