1. A quality prognostics system for manufacturing processes, comprising: conjecture modeling means for using a set of current input data of a production tool to conjecture a conjecture value for a product lot currently manufactured in said production tool, wherein said conjecture modeling means is built in accordance with a conjecture method, and said conjecture method is selected from the group consisting of a first neural network technique, a fuzzy logic technique and a stepwise regression technique; and prediction modeling means for using said conjecture value of said product lot together with at least two actual measurement values of at least two previous product lots to predict a prediction value for a next product lot, wherein said at least two previous product lots are respectively produced in at least two production cycles, and said at least two actual measurement values are one-to-one corresponding to said at least two previous product lots respectively, and said prediction modeling means is built in accordance with a prediction method, and said prediction method is selected from the group consisting of a weighted moving average technique and a second neural network technique.