<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (August 2011) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (August 2011) (Learn how and when to remove this template message) </Td> </Tr> <P> A scientific control is an experiment or observation designed to minimize the effects of variables other than the independent variable . This increases the reliability of the results, often through a comparison between control measurements and the other measurements . Scientific controls are a part of the scientific method . </P> <P> Controls eliminate alternate explanations of experimental results, especially experimental errors and experimenter bias . Many controls are specific to the type of experiment being performed, as in the molecular markers used in SDS - PAGE experiments, and may simply have the purpose of ensuring that the equipment is working properly . The selection and use of proper controls to ensure that experimental results are valid (for example, absence of confounding variables) can be very difficult . Control measurements may also be used for other purposes: for example, a measurement of a microphone's background noise in the absence of a signal allows the noise to be subtracted from later measurements of the signal, thus producing a processed signal of higher quality . </P>

Why is it necessary to control variables in an experiment