>>4
It depends on what kind of prediction. Long-term predictions can't be reliable, but very short-term (second/minute-long) ones can be accurate with a over 50% success rate. The trick is getting the data fast enough and processing it fast enough (hence people buying servers as close as possible to stock exchanges and even resorting to using FPGA's to do the processing). This is how certain big trading companies are making their money, constantly selling/buying the same stocks, predicting very small changes/fluctuations.