Reading through Hanon discussions, most of the arguments were anecdotal:
'i did X and it worked for me';
'i didn't do X and i came to no harm';
'Z did X and he was very good'.
Have there been any methodical studies comparing different teaching approaches?
I'm think of something like:
'We took 30 students of similar ability, and randomly assigned 15 to practise repertoire
for an hour a day;15 to practise Hanon for 15 mins, and repertoire for 45 mins.
After 3 months we compared their progress.'
It would appear reasonably straightforward to set up such a series of experiments,
and that having a such an evidence base would be generally useful for educators
but i can't find any sign of such research.