Is technical analysis the Holy Grail for investors? Or is it just tea leaf reading?
One of the most biting criticisms of technical analysis is the idea that there's no way past prices can be a crystal ball for future prices.
It works but it is getting harder and harder because more people use software and computers to analyze prices and as a result anomalies that give rise to profitable patterns and trading rules are eliminated at higher rates than before.
I found many answers in the recent book by M. Harris "Fooled by Technical Analysis"
In the book Harris talks about what works and what does not. He makes some interesting arguments against the constant use of backtesting and warns about the dangers of data-mining bias. Format is web based but worth reading it.