Well generally speaking, we all enter a strategy under at least one assumption, that future price will belong to the same distribution family as past price. So there is always an advantage to testing over longer periods since the future price might take on characteristics of some older version of the distribution. On the other hand, if you are optimizing your parameters, then you might want to focus on more local data to catch subtle changes which can have a huge effect on performance. So the way I think about it, back-test over as much time as possible, and optimize over as little time as possible. (of course the optimization window should be tuned as well.)
I'd say Mayweather pretty much knew exactly the outcome of every match before he started. It's not impossible.
I can't possibly imagine doing a trade that wasn't fully automated. Would make me a nervous wreck. Major respect to those who can trade manually. Seriously, I would just explode after a few hours.
That's fair and maybe for you individually your make up or something mentally means you really can't do it no matter what. But I would just point back to your earlier comment regarding mayweather. It's the same way with manual trading (not to mention you can also have automation to back you up as well). You know the ultimate outcome or at least the probability is on your side before placing the trade. Sure, like mayweather you may take some punches or even lose some rounds, but you know in the end probability favors you to win the fight.
totally. my best friends are law of large numbers, positive mean return, and lots of opportunities, is the way i see it.