Reliability
Several studies have estimated the internal consistency of the SDS; sample sizes ranged from 100 to 225. Alphas ranging from 0.75 to 0.95 have been reported (9–11). Zung and Zung reviewed studies using the SDS with elderly people; in four such studies alphas ranged from 0.59 to 0.87 (12, Table 2). Split-half reliability was estimated at 0.73 (4, p225) and at 0.81 (13, Table 2). Mean item-total correlations were 0.36 in samples of New York drug addicts and Nigerian students; alphas were 0.75 and 0.79 (11, p28). The one-year test-retest correlation was 0.61 in 279 older subjects (14, p179).