I appreciate that you want to categorise and be scientific - no problem.
However without a doubt the main cause of slow play is a players mind set,not their ability.
There is a clear reluctance to pressure group's ahead that are playing slowly.
Example, I was recently in a club competition...
Its a great topic of conversation - someone always knows someone who does not know the rules. I thought I did but then took the quiz on the R&A website. We all need to improve our knowledge since, if we do not play by the rules of the game, then we are not actually playing golf. We should not...