In a system for data processing by use of a processing language controlled by a job control program for coupling a file having at least one data item to a data set, a method of standardizing data items in an existing program stored in a storage and having a procedure part and a data definition part is disclosed. The names of two data items including the source and destination items of moving in the transfer designated in a plurality of data transfer instructions in an existing program are stored in the storage, and the equivalence number attached to the name of the transfer source item of each of the plurality of data transfer instructions is attached to the name of the transfer destination item. If a plurality of files assigned in the data definition part are defined in a single data set, the file equivalence number is attached to the plurality of files in the storage. The starting position and length of the data items are stored in the storage on the basis of the data length for the data item name in the data definition part, and the data item name is changed on the basis of the relative positions therebetween and the equivalence number thereof.
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