To reduce variables in any type of experiment it is advisable to include both positive and negative controls in the experimental design.
Negative controls are particular samples included in the experiment that are treated the same as all the other samples, but are not expected to change due to any variable in the experiment. The positive control sample will show an expected result, helping the scientist to understand that the experiment was performed properly. Some controls are specific to the type of experiment being performed such as molecular weight standards used in protein or DNA gel electrophoresis, i.e. SDS-PAGE or agarose gel electrophoresis. The proper selection and use of controls ensures that experimental results are valid and saves valuable time.