Rule Definition
Using indexes to address a table is more efficient than using subscripts since the index already contains the displacement from the start of the table and does not have to be calculated at run time. Subscripts, on the other hand, contain an occurrence number that must be converted to a displacement value at run time before it can be used. When using subscripts to address a table, use a binary (COMP) signed data item with eight or fewer digits (for example, using PICTURE S9(8) COMP for the data item). This will allow fullword arithmetic to be used during the calculations. Additionally, in some cases, using four or fewer digits for the data item may also offer some added reduction in CPU time since halfword arithmetic can be used.

Remediation
Prefer to use index rather than subscript. Add an INDEXED clause in table definition and use this index.

Violation Code Sample

01 DIRECT-TAX-RATE. 
02 TAX-RATE PIC 99 OCCURS 5 TIMES. 
01  TX-INDEX PIC X(10).
... 
SET TX-INDEX TO 4. 
MOVE TAX-RATE(TX-INDEX) TO SEC-RATE.

Fixed Code Sample

01 DIRECT-TAX-RATE. 
02 TAX-RATE PIC 99 OCCURS 5 TIMES INDEXED BY TX-INDEX. 
... 
SET TX-INDEX TO 4. 
MOVE TAX-RATE(TX-INDEX) TO SEC-RATE.

IBM Cobol - Best Practices and Recommendations