Inspectors are unable to say whether a long-running training programme for new headteachers has helped raise school standards - because the scheme was not monitored properly.
The Office for Standards in Education says the quality of training offered by the Headlamp programme was good, and often very good. But some training providers did not tailor courses to suit heads and their schools.
And they failed to monitor if, or how much, headteachers' leadership skills improved as a result of training, making it difficult to assess the course's effectiveness.
Inspectors had raised similar concerns in earlier inspections, and note that little progress has been made. Around 11,600 new heads have completed or are still taking part in Headlamp since its introduction in April 1995.
The programme has been revised by the National College for School Leadership and relaunched this month as the Headteacher Induction Programme (Hip). So far, 700 heads have requested application forms for the new scheme, which gives them pound;2,500 to spend on training to improve their leadership skills.
Heather Du Quesnay, chief executive of the college, said it has improved its assessment of what heads need from the course and reduced the number of training providers from 160 to 20 to make it easier to monitor the quality and consistency of provision. An independent evaluation of the scheme is also planned.
Inspectors examined the work of six providers. They also found that mentoring support, usually provided by experienced heads, was not helpful because they proved reluctant to act as "critical friends" - so new colleagues were not challenged enough.
Training for newly appointed headteachers (Headlamp), see www.ofsted.gov.ukpublications and www.ncsl.org.uk