The Digit Symbol test, however, is

The Digit Symbol test, however, is a speeded test and the split-half technique for computing reliability is inappropriate. It was not feasible to give an alternate form of the Digit Symbol test to the subjects in the standardization population since these people had already submitted to the full scale and might not react kindly to the idea of additional testing. Consequently, a special study was undertaken to estimate the reliability of the Digit Symbol test