1. A method of extracting abnormal factors in a processing operation for a diffusion process in a semiconductor manufacturing plant for producing electrical products, comprising the steps of: collecting and storing data on product quality including yield and electric characteristics; collecting and storing data on the processing operation which affects the product quality including producing apparatus histories, producing conditions and in-line measurement values; processing the stored data in preparation for an analysis, including determining when the producing apparatus histories data has a statistically insufficient amount of production information data for a product and preparing a correspondingly corrected producing apparatus histories data; and analyzing a causal relation between the data on product quality and the processing operation data which affects the product quality with a multistage multivariate analysis procedure including extracting a predetermined number of production operations from the corrected apparatus history data to form a first group of a specific number and for forming a second group of production operations of the producing apparatus histories data of apparatus having a statistically adequate amount of production history data, wherein a first stage multivariate analysis procedure reads out data from the first and second groups of production operations using the yield data as object variables to extract a limited number of abnormal factors.