from the Creery sister and the Soal-Goldney experiments were proven to be fraudulent, one of the subjects from the Smith-Blackburn experiments confessed to fraud, the Brugmans experiment, the experiments by John Edgar Coover and those conducted by Joseph Gaither Pratt and Helmut Schmidt had flaws in the design of the experiments, did not rule out the possibility of sensory cues or trickery and have not been replicated. According to critics, psi is negatively defined as any effect that cannot be currently explained in terms of chance or normal causes and this is a fallacy as it encourages parapsychologists into using any peculiarity in the data as a characteristic of psi. Parapsychologists have admitted it is impossible to eliminate the possibility of non-paranormal causes in their experiments. There is no independent method to indicate the presence or absence of psi. Persi Diaconis has written that the controls in parapsychological experiments are often loose with possibilities of subject cheating and unconscious sensory cues. The existence of parapsychological phenomena and the scientific validity of parapsychological research is disputed by independent evaluators and researchers. In 1988, the U.S. National Academy of Sciences published a report on the subject that concluded that "no scientific justification from research conducted over a period of 130 years for the existence of parapsychological phenomena." No accepted theory of parapsychology currently exists, and many competing and often conflicting models have been advocated by different parapsychologists in an attempt to explain reported paranormal phenomena. Terence Hines in his book Pseudoscience and the Paranormal (2003) wrote "Many theories have been proposed by parapsychologists to explain how psi takes place. To skeptics, such theory building seems premature, as the phenomena to be explained by the theories have yet to be demonstrated convincingly." Skeptics such as Antony Flew have cited the lack of such a theory as their reason for rejecting parapsychology. In 1998, physics professor Michael W. Friedlander noted that parapsychology has "failed to produce any clear evidence for the existence of anomalous effects that require us to go beyond the known region of science." Philosopher and skeptic Robert Todd Carroll has written research in parapsychology has been characterized by "deception, fraud, and incompetence in setting up properly controlled experiments and evaluating statistical data." The psychologist Ray Hyman has pointed out that some parapsychologists such as Dick Bierman, Walter Lucadou, J. E. Kennedy, and Robert Jahn have admitted the evidence for psi is