In reply to bigrob:
First you need to refine/identify what you mean by young people with challenging behaviour. Risk factors and behaviour management techniques for young people who have autism are very different from those used with young people who have committed offences for example.
Secondly, I would be very sceptical of any study/organisation/person that categorically states that they can prove their intervention had a life changing impact on a young person. For example, any organisation which claims to reduce reoffending rates I would ask 'how do you know?'. Young people have many different services and influences on their lives from youth offending teams and social services to family friends, even television. you can not scientifically prove that you are the one thing that has stopped them reoffending.
So, look at risk factors for behaviour management and theories of change. in offending behaviour look up disistance theory, for autism speak to specialists about communication styles etc. It is possible to prove that interventions have increased someone's self efficacy (through observation and testimonial evidence) or educational attainment (if they gain new skills or even a qualification) which can contribute to improved outcomes for young people.
This approach also gets round post intervention tracking which has confidentiality, safeguarding and logistical/paperwork issues.
OK, I think that's enough waffle! I hope some of it makes sense! Which uni are you at? I'm not trying to put you off the work (think it can be brilliant) or the study, just trying to stop you falling into a hole that I've seen many others end up in.