Extraneous variables prevent the experimenter from making strong conclusions and must be reduced or eliminated to achieve the best experimentation...

Experiments are conducted to study the effect of variations of an independent variable on the dependent variable. However, there are variables that are not actually varied by the experimenter and yet they affect the experimental outcome. Hence such variables, also known as the extraneous variables, are undesirable in an experimental study and need to be minimized and/or removed. One way to do that, and to achieve the best experimental design, is by the use of experimental controls. 

Experimental controls can be introduced at several stages in an experimental study. They could be used at the stage of sample (or data) procurement, experimental setting, measurement, control samples, control groups, etc. For example, to study the effect of the amount of sugar on the sweetness of a cup of tea, a good control is testing a cup of tea without any sugar in it. Another control could be using the same cup size (or volume of tea). Yet another control would be using the same person to test the sweetness of each cup.

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