Any unit of measurement would have been arbitrary, and twelfths at least lets you refer to the first third, quarter or half. No idea whether that was the rationale, but I don't think any other division would have seriously affected the point the article is making.

Well, if the point of the study would change with the choice of the units, then the study isn't that good, is it? :)

I'm not arguing the point. I'm arguing, that using twelfths has not utility over using per cent, for example, but is counter-intuitive.

Three act structure with each act divided into quarters.

Though, to be honest, three acts is kinda misleading since most second acts have a big turning point in the middle. So while yeah, there's a beginning, middle, and end, that middle is really two acts.

Good point. Splitting it up into quarters would make more sense.

12 parts be quarters split into three as easily as thirds split into four.

Though in any case it would be nice for the article to state why that division was used (even if the reason is completely arbitrary - at least then we'd know to stop guessing!).