Thanks for the info. And I thought 'momentum' was the 4th FF factor
Yeah, momentum tends to get thrown in the mix as a factor, but really, it's just a price trend. You'd be wise to not jump head-long into negative momentum stocks or be in a rush to sell positive momentum ones. But building an entire fund around it is something else entirely-- 200% plus turnover, etc. that's hard to reconcile with passive investing. Most good indexes with hold ranges/buffer zones do about as much with momentum as needed.
Broad based factors are better looked at in terms of some sort of price or cash-flow variable. Profits, earnings, book value, company size, fixed income maturity, stability of issuer--these are general and intuitive characteristics that should have some ability to segment different types of investments and associated risk/return. And that's all we're looking for from factors: some small improvement in our ability to understand the seemingly random market movements that allow us to better plan and invest our money.