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Reproducibility is one of the main principles of the scientific method, and refers to the ability of a test or experiment to be accurately reproduced, or replicated, by someone else working independently.

Reproducibility is different from repeatability, which measures the success rate in successive experiments, possibly conducted by the same experimenters. While repeatability of scientific experiments is desirable, it is not considered necessary to establish the scientific validity of a theory. For example, the cloning of animals is difficult to repeat, but has been reproduced by various teams working independently, and is a well established research domain. One failed cloning does not mean that the theory is wrong or unscientific. Repeatability is often low in protosciences.

The results of an experiment performed by a particular researcher or group of researchers are generally evaluated by other independent researchers by reproducing the original experiment. They repeat the same experiment themselves, based on the original experimental description, and see if their experiment gives similar results to those reported by the original group. The result values are said to be commensurate if they are obtained (in distinct experimental trials) according to the same reproducible experimental description and procedure.

The basic idea can be seen in Aristotle's dictum that there is no scientific knowledge of the individual, where the word used for individual in Greek had the connotation of the idiosyncratic, or wholly isolated occurrence. Thus all knowledge, all science, necessarily involves the formation of general concepts and the invocation of their corresponding symbols in language (cf. Turner).

In March 1989, University of Utah chemists Stanley Pons and Martin Fleischmann reported the production of excess heat that could only be explained by a nuclear process. The report was astounding given the simplicity of the equipment: it was essentially an electrolysis cell containing heavy water and a palladium cathode which rapidly absorbed the deuterium produced during electrolysis. The news media reported on the experiments widely, and it was a front-page item on many newspapers around the world. Over the next several months others tried to replicate the experiment, but were unsuccessful. At the end of May the US Energy Research Advisory Board found the evidence to be unconvincing, and cold fusion was dismissed as pseudoscience. Later on, successful replications by independent teams were reported in peer reviewed scientific journals, and, although the effect is not considered fully repeatable, the field eventually gained some scientific recognition.^[1]

In the 1930's the Austrian scientist Wilhelm Reich claimed to have discovered a physical energy he called "orgone," and which he said existed in the atmosphere and in all living matter. He developed instruments to detect and harness this energy that he said could be used to treat illness or control the weather. His views were not accepted by the mainstream scientific community; in fact, he was vilified for his claims. In the early 1940's Reich encouraged Albert Einstein to test an orgone accumulator, which Einstein did, but he disagreed on the interpretation of the results. In 2001, Canadian researchers Paulo Correa and Alexandra Correa claimed to have successfully reproduced the experiment. But Martin Gardner's book, Fads and Fallacies in the Name of Science debunks orgone energy.

Nikola Tesla claimed as early as 1899 to have used a high frequency current to light gas-filled lamps from over 25 miles away without using wires. In 1904 he built Wardenclyffe Tower on Long Island to demonstrate means to send and receive power without connecting wires. The facility was never fully operational and was not completed, supposedly due to economic problems. Tesla's experiments have never been replicated.

• Turner, William (1903), History of Philosophy, Ginn and Company, Boston, MA, Etext. See especially: "Aristotle".
• Definition (PDF)

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• Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results; appendix D
• Definition of reproducibility in the IUPAC Gold Book
• Detailed article on Reproducibility
• Journal of Irreproducible Results (Spoof)