Rhine took precautions in response to criticisms of his methods, he was unable to find any high-scoring subjects. Another criticism, made by chemist Irving Langmuir, among others, was one of selective reporting. Langmuir stated that Rhine did not report scores of subjects that he suspected were intentionally guessing wrong, and that this, he felt, biased the statistical results higher than they should have been. Rhine and his colleagues attempted to address these criticisms through new experiments described in the book Extrasensory Perception After Sixty Years (1940). Rhine described three experiments the Pearce-Pratt experiment, the Pratt-Woodruff experiment and the Ownbey-Zirkle series which he believed demonstrated ESP. However, C. E. M. Hansel wrote "it is now known that each experiment contained serious flaws that escaped notice in the examination made by the authors of Extra-Sensory Perception After Sixty Years". Joseph Gaither Pratt was the co-experimenter in the Pearce-Pratt and Pratt-Woodruff experiments at the Duke campus. Hansel visited the campus where the experiments took place and discovered the results could have originated through the use of a trick so could not be regarded as supplying evidence for ESP. In 1957, Rhine and Joseph Gaither Pratt wrote Parapsychology: Frontier Science of the Mind. Because of the methodological problems, parapsychologists no longer utilize card-guessing studies. Rhine's experiments into psychokinesis (PK) were also criticized. John Sladek wrote: His research used dice, with subjects 'willing' them to fall a certain way. Not only can dice be drilled, shaved, falsely numbered and manipulated, but even straight dice often show bias in the long run. Casinos for this reason retire dice often, but at Duke, subjects continued to try for the same effect on the same dice over long experimental runs. Not surprisingly, PK appeared at Duke and nowhere else. The Ownbey-Zirkle ESP experiment at Duke was criticized by parapsychologists and skeptics. Ownbey would attempt to send ESP symbols to Zirkle who would guess what they were. The pair were placed in adjacent rooms unable to see each other and an electric fan was used to prevent the pair communicating by sensory cues. Ownbey tapped a telegraph key to Zirkle to inform him when she was trying to send him a symbol. The door separating the two rooms was open during the experiment, and after each guess Zirkle would call out his guess to Ownbey who recorded his choice. Critics pointed out the experiment was flawed as Ownbey acted