Control Experiments

A control refers to a standard or reference treatment or group in an experiment. 

It is the same as the experimental (test) group, except that it lacks the one variable being manipulated by the experimenter. 

Controls are used to demonstrate that the response in the test group is due to a specific variable (e.g. temperature). 

The control undergoes the same preparation, experimental conditions, observations, measurements and analysis as the test group. This helps ensure that responses observed in the treatment groups can be reliably interpreted. 

Data-gathering investigations sometimes have the control built into the investigation. For example, when testing water quality, researchers would sample both upstream and downstream of a pollution source. The upstream sample acts as the control (reference). 

Another example is testing the effect of a certain nutrient on microbial growth. All agar plates are prepared in the same way, but the control plate does not have the test nutrient applied. 

Each plate is inoculated from the same stock solution, incubated under the same conditions, and examined under the same conditions, and examined at the same set periods. The control plate sets the baseline; any growth above that seen on the control plate is attributed to the nutrient.