[creeble]

It very much "depends". I'll give you an example.

A friend had a technique that worked very well on the NASDAQ 100 with roughly those BT criteria. One problem was overlooked: The NASDAQ 100 changes over that time. So you aren't necessarily investing in the same securities in year 6 as you were in year 5, even though you're investing in the same "top 100". IOW, backtesting in this particular case had future knowledge that wasn't easy to see.

When he started (paper) trading in the real NASDAQ 100, the returns were negative. In that he was lucky: They could have been positive for some time, until the constituent stocks changed.