In designing an experiment, there is a need to change some parameters in order to get different results to justify a certain claim or hypothesis. Groupings are made based on the parameter being set. These can be the experimental and the controlled group.
The experimental group is where the researcher aims to seek results of the changes in the action of a particular experiment. The control group on the other hand is a group separated from the experimental set up. This group however is treated exactly like those of the experimental group. The only difference is that in the control group, the variable in the experimental group is not present.
The importance of the control group is employed in an experiment to see if there is an observable result when the variables are removed. This is also useful when the samples are complex and harder to determine.
Consider this illustration as an example:
Hypothesis: Cells exposed to toluene become cancer cells
Experimental group: Cells that are exposed to toluene
Controlled group: Cells that are not exposed to toluene
A control group is crucial in a lab because it provides validity to your results. It allows whoever is reading your lab report to know that there were no problems with the original and gives them something to compare to with your experimental group.
For example, when testing the effectiveness of amoxicillin on E. coli, you would need at least two Petri dishes. One would display the growth of the bacterial colonies without any antibiotics to show that your original bacteria were healthy and alive. After that, you could have as many different Petri dishes as you wanted displaying various amounts of amoxicillin to test for its effect on E. coli.