Tiggy, I read about a research when they calculated correlation with parents involvement quite clearly. The subject of the research were young musicians, participants of the international competitions. The researchers asked parents about family involvement in studies - say, if the child is just dropped at teacher's door, or parent/guardian goes with the child, waits in the corridor or sits in the classroom, records the homework, checks/guides doing the homework and so forth. All that was compared to the children's performance in those competitions.
So, it is possible to do quite meaty statistical research in this field.