<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> <P> For example, if a researcher feeds an experimental artificial sweetener to sixty laboratory rats and observes that ten of them subsequently become sick, the underlying cause could be the sweetener itself or something unrelated . Other variables, which may not be readily obvious, may interfere with the experimental design . For instance, the artificial sweetener might be mixed with a dilutant and it might be the dilutant which causes the effect . To control for the effect of the dilutant, another treatment is added which is the dilutant alone . Now the experiment is controlled for the dilutant and the experimenter can distinguish between sweetener, dilutant and non-treatment . Controls are most often necessary where a confounding factor cannot easily be separated from the primary treatments . For example, it may be necessary to use a tractor to spread fertilizer where there is no other practicable way to spread fertilizer . The simplest solution is to have a treatment where a tractor is driven over plots without spreading fertilizer and in that way the effects of tractor traffic are controlled . </P> <P> The simplest types of control are negative and positive controls, and both are found in many different types of experiments . These two controls, when both are successful, are usually sufficient to eliminate most potential confounding variables: it means that the experiment produces a negative result when a negative result is expected, and a positive result when a positive result is expected . </P>

What is the name for something that changes during an experiment