Does psi exist? Comments on Milton and Wiseman's (1999) meta-analysis of ganzfeld research.
J. Milton and R. Wiseman (1999) attempted to replicate D. Bem and C. Honorton's (1994) meta-analysis, which yielded evidence that the ganzfeld is a suitable method for demonstrating anomalous communication. Using a database of 30 ganzfeld and autoganzfeld studies, Milton and Wiseman's meta-analysis yielded an effect size (ES) of only 0.013 (Stouffer Z = 0.70, p = .24, one-tailed). Thus they failed to replicate Bem and Honorton's finding (ES = 0.162, Stouffer Z = 2.52, p = 5.90 x 10(-3), one-tailed). The authors conducted stepwise performance comparisons between all available databases of ganzfeld research, which were argued not to be lacking in quality. Larger aggregates of such studies were formed, including a database comprising 79 ganzfeld-autoganzfeld studies (ES = 0.138, Stouffer Z = 5.66, p = 7.78 x 10(-9)). Thus Bem and Honorton's positive conclusion was confirmed. More accurate population parameters for the ganzfeld and autoganzfeld domains were calculated. Significant bidirectional psi effects were also found in all databases. The ganzfeld appears to be a replicable technique for producing psi effects in the laboratory.