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, a stepwise regression technique and other technologies having the conjecturing capability; and prediction modeling means for using said conjecture value of said product lot together with at least one actual measurement value of at least one previous product lot to predict a prediction value for a next product lot, wherein said at least one previous product lot is respectively produced in at least one production cycle, and said at least one actual measurement value is one-to-one corresponding to said at least one previous product lot, 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, a second neural network technique and other algorithms having the prediction capability.