Nobody knows how many people drop out of British post-16 education, according to unpublished research by exam watchdogs.
A survey, finished last month by staff at the old National Council for Vocational Qualifications, anal-ysed the many studies into student drop-out rates.
Despite all this effort, the researchers found no complete picture in an area vital to the Government's drive to promote a lifelong learning strategy.
Particular problems were found in vocational education, where little is known about qualifications obtained at work - a fast-expanding area.
Researchers from the NCVQ - now part of the Government's new exam superbody, the Qualifications and Curriculum Authority - recommend that all future research should be vetted to stop duplication and to ensure that findings contribute to a real understanding of what is going on in the country.
And a long-term aim should be to establish ways of collecting information which allow drop-out rates to be compared, says the report.
"Although many studies have been carried out over a number of years into non-completion and associated areas of research, none of them appears to provide a complete picture," it concludes.
The studies looked at different timescales, different sets of qualifications and different sectors. This, the report says, makes it difficult to calculate drop-out rates.
"The information that is available appears to be primarily about qualifications obtained through educational establishments and there appears to be little or no information about candidates assessed for national vocational qualifications in the workplace."
The admission from the NCVQ that little is known about NVQ trainees follows intense criticism of the system. About 16,000 fall outside the NVQ system. Hundred of NVQs have never been awarded.
The report adds: "There is little information on vocational qualifications in general or NVQs in particular.
"Nor is there much information on qualifications attained through the workplace, although there is information on youth traineeships and apprenticeships that lead to formal qualifications.
"Information about workplace-based qualifications including NVQs is important, not least because an increase in the number of people obtaining qualifications in the workplace contributes to achievement of the lifetime education and training targets."
The NCVQ study into the available research was prompted by Sir Ron Dearing's recommendation that more research was needed into drop-out rates.
But it comes as members of the powerful Commons select committee on education start their review of further education - looking particularly at franchised courses run in the workplace.
Concerns have focused on the question of the value of colleges' involvement with employers - and the extent to which public money is simply replacing private training.
The report says: "Careful consideration will need to be given to the definitions used for the new research and for any future routine monitoring to ensure that problems of inconsistency are overcome."