In a plasma processing method for monitoring data, first and second measurement data are obtained; and a first and a second model are formulated based on the first and the second measurement data. Further, third measurement data is obtained; and weight factors are obtained by setting the third measurement data as weighted measurement data wherein the weighted measurement data is obtained by multiplying each of the first and the second measurement data by one of the weight factors to produce first and second weighted data and summing the thus produced first and the second weighted data. Therefore, a third model is formulated by multiplying each of the first and the second model by one of the weight factors to produce first and second weighted models, and summing the thus produced first and the second weighted models.