Ofqual's research on the validity of qualifications
Research papers by and for Ofqual on the validity of qualifications and assessments and what improvements can be made.
Validity is about whether exam results are an accurate measure of what a qualification tests: what students know and can do. For an exam to be valid it must cover the important aspects of the syllabus and the process for setting standards must be appropriate. You might also ask whether the end result of the qualification is valid for the intended use. If the main use of a geography GCSE is to allow a student to carry on and study geography at A level, does it actually do that?
Validity is often considered hand-in-hand with reliability, and you may find this article on assessment reliability and validity useful in understanding the difference between validity and reliability.