It seems a common theme with traders that they find an edge in the market which works for a while, but then the market 'changes' and the edge disappears and starts losing money. This seems odd to me; clearly the market is sometimes going up, sometimes going down, and sometimes going sideways; is sometimes volatile and sometimes not; but is there more to it than that? Surely an 'edge' should be able to cope with such things, as they clearly happen quite frequently and many traders say their systems have been tested over 5 or 10 years? I have never seen anyone explain specifically how the market 'changed' in some mysterious way, other than to make their edge non-profitable. It seems more likely to me that the edge never really existed, and that the trader had a promising backtest and was then 'fooled by randomness' as Taleb might say. Perhaps some of the experienced traders can throw some light on it to a rookie like me.