<P> In the centuries that followed, people who applied the scientific method in different areas made important advances and discoveries . For example, Galileo Galilei (1564 - 1642) accurately measured time and experimented to make accurate measurements and conclusions about the speed of a falling body . Antoine Lavoisier (1743 - 1794), a French chemist, used experiment to describe new areas, such as combustion and biochemistry and to develop the theory of conservation of mass (matter). Louis Pasteur (1822 - 1895) used the scientific method to disprove the prevailing theory of spontaneous generation and to develop the germ theory of disease . Because of the importance of controlling potentially confounding variables, the use of well - designed laboratory experiments is preferred when possible . </P> <P> A considerable amount of progress on the design and analysis of experiments occurred in the early 20th century, with contributions from statisticians such as Ronald Fisher (1890 - 1962), Jerzy Neyman (1894 - 1981), Oscar Kempthorne (1919 - 2000), Gertrude Mary Cox (1900 - 1978), and William Gemmell Cochran (1909 - 1980), among others . </P> <P> Experiments might be categorized according to a number of dimensions, depending upon professional norms and standards in different fields of study . In some disciplines (e.g., psychology or political science), a' true experiment' is a method of social research in which there are two kinds of variables . The independent variable is manipulated by the experimenter, and the dependent variable is measured . The signifying characteristic of a true experiment is that it randomly allocates the subjects to neutralize experimenter bias, and ensures, over a large number of iterations of the experiment, that it controls for all confounding factors . </P> <P> A controlled experiment often compares the results obtained from experimental samples against control samples, which are practically identical to the experimental sample except for the one aspect whose effect is being tested (the independent variable). A good example would be a drug trial . The sample or group receiving the drug would be the experimental group (treatment group); and the one receiving the placebo or regular treatment would be the control one . In many laboratory experiments it is good practice to have several replicate samples for the test being performed and have both a positive control and a negative control . The results from replicate samples can often be averaged, or if one of the replicates is obviously inconsistent with the results from the other samples, it can be discarded as being the result of an experimental error (some step of the test procedure may have been mistakenly omitted for that sample). Most often, tests are done in duplicate or triplicate . A positive control is a procedure similar to the actual experimental test but is known from previous experience to give a positive result . A negative control is known to give a negative result . The positive control confirms that the basic conditions of the experiment were able to produce a positive result, even if none of the actual experimental samples produce a positive result . The negative control demonstrates the base - line result obtained when a test does not produce a measurable positive result . Most often the value of the negative control is treated as a "background" value to subtract from the test sample results . Sometimes the positive control takes the quadrant of a standard curve . </P>

What is an experiment that only tests one variable at a time