There are several common criticisms of some or all of the ganzfeld experiments:
Isolation — Richard Wiseman and others argue that not all of the studies used soundproof rooms, so it is possible that when videos were playing, the experimenter could have heard it, and later given involuntary cues to the receiver during the selection process. It could even have been possible that the receiver themselves could hear the video.[21]
Randomization — When subjects are asked to choose from a variety of selections, there is an inherent bias to choose the first selection they are shown. If the order in which they are shown the selections is randomized each time, this bias will be averaged out. The randomization procedures used in the experiment have been criticized for not randomizing satisfactorily.[34]
The psi assumption — The assumption that any statistical deviation from chance is evidence for telepathy is highly controversial. Strictly speaking, a deviation from chance is only evidence that either this was a rare, statistically unlikely occurrence that happened by chance, or something was causing a deviation from chance. Flaws in the experimental design are a common cause of this, and so the assumption that it must be telepathy is fallacious.[35]
Writing in 1985, C. E. M. Hansel discovered weaknesses in the design and possibilities of sensory leakage in the ganzfeld experiments reported by Carl Sargent and other parapsychologists. Hansel concluded the ganzfeld studies had not been independently replicated and that "ESP is no nearer to being established than it was a hundred years ago."[36]
Terence Hines wrote in 2003 that the ganzfeld studies could not be said to provide evidence for psi as the alleged evidence disappears as the tightness of experimental controls is increased. As research progresses variables in science become clearer as more studies are published that describe under what specific condition the particular effect can be demonstrated. This is in opposition to the ganzfeld studies. According to Hines, there was "no clear way to obtain results showing any psychic phenomenon reliably" and that "the most reasonable conclusion" was that the effect did not exist and had never existed.[37]
In a 2007 review, Ray Hyman wrote that parapsychologists agree they have no positive theory of psi as it is negatively defined as any effect that cannot be currently explained in terms of chance or normal causes. Hyman saw this as a fallacy, as it encouraged parapsychologists to use any peculiarity in the data as a characteristic of psi. Hyman also wrote that parapsychologists have admitted it is impossible to eliminate the possibility of non-paranormal causes in the ganzfeld experiment. There is no independent method to indicate the presence or absence of psi.[14]
Until parapsychologists can provide a positive way to indicate the presence of psi, the different effect sizes that occur in experiments are just as likely to result from many different things rather than one thing called psi. Indeed given the obvious instability and elusiveness of the findings, the best guess might very well be that we are dealing with a variety of Murphy's Law rather than a revolutionary anomaly called psi.
—Ray Hyman, Evaluating Parapsychological Claims, 2007[14]
In a 2013 podcast, Brian Dunning reviewed the flaws of the ganzfeld studies and came to the conclusion the technique had failed as evidence for psi and interest in ganzfeld has declined.[38]
