A plurality of lots of wafers, each lot of wafers having a lot number and each wafer of each lot having at least one test parameter generated by performing at least one wafer test item stored in a database, are divided into a high yield group and a low yield group. By analyzing the wafer test parameters of the wafers in the high yield group, a first standard value within a first range is obtained. A first comparison step is then performed to compare each wafer test parameter of each lot in the low yield group with the first standard value and delete lot numbers of lots with wafer test parameters within the first range. Finally, a first amount of residual lots in the low yield group is determined. In response to the first amount of residual lots in the low yield group not equaling to zero, a first searching step is performed to which item of sample test items, in-line QC items and process step items is related to the wafer test item of each residual lot in the low yield group in the database.