It is perhaps education's equivalent to the search for the Holy Grail - or the answer to life, the universe and everything.
Grappled with by teachers and educationists for millennia, the perennial question goes a bit like this: if you could change one thing about the way our schooling system is run, what would it be?
Now, what is believed to be the largest ever educational research study - covering more than 80 million pupils and bringing together more than 50,000 smaller studies - has come up with the answer.
Perhaps unsurprisingly, it turns out the solution is relatively simple: the best way to get higher achievement is to improve the level of interaction between pupils and their teachers. Not exactly rocket science, some might say.
Improving feedback between teachers and their pupils, giving both parties the information they need to improve and making sure that children are sufficiently stretched is fundamental, according to the research, which took a mere 15 years to compile.
Raising teachers' game in this way will do more for pupils' prospects than could ever be achieved by changing the organisation of the schools they attend or the size of classes, the study found.
Professor John Hattie, of Auckland University, has spent a decade and a half studying research on the effects of every type of educational intervention used in classrooms throughout the English-speaking world.
The higher echelons of his league table of 138 types of reform - ranked in terms of their effect on raising pupil achievement - are dominated by programmes that concentrate on improving pupil-teacher interaction.
Vital feedback, pages 22-23.