During an experiment, data from an experimental group are compared with the data of a control group. These two sets are identical in all aspects except for the independent variable that the researcher changes in the experimental group to observe how they affect the individuals. This variable keeps constant in the control group.
The experimental group is the one that receives the experimental procedure, with changes in the independent variable. There can be several experimental groups.
In the control group, the variable measured keeps constant, not influencing the results. This isolates the effect of the independent variable on the experiment and helps to find alternative explanations to the different results.
In the exposed example:
Control group: Certain amounts of fishes exposed to a normal number of light hours per day. For example, if under natural conditions in its original environment the species is exposed to 12 hour light, then the control group should be exposed constantly to 12 hours light a day to maintain its normal reproductive levels.
Experimental group: Certain amounts of fishes, equal to the control group, exposed to a different number of light hours per day. For example, there might be 5 experimental groups: one of them exposed to 4 hours light, a second group exposed to 8 hours light, the third group exposed to 12 hours light (The same as the control group), the fourth group exposed to 16 hours light, and the fifth group exposed to 20 hours light.
Except for light, the rest of the variables should be the same for all the groups.
