In one experiment, scientists raised mice in germ-free conditions so the mice lacked intestinal microbes. The mice were fed a low-fat diet rich in the complex plant polysaccharides, such as cellulose, that are often called fiber. When the mice were 12 weeks old, the scientists transplanted the microbial community from the intestine of a single "donor" mouse into all of the germ-free mice. Then they divided the mice randomly into two groups and fed each group a different diet. Group 1 (the control group) continued to eat a low-fat, high-fiber diet. Group 2 (the experimental group) ate a high-fat, high-sugar diet

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In any experiment independent variable is the factor that is being tested. It is controlled by the experimenter and its effect is checked on the dependent variable. Here, the effect of the diet on mice microbial community is being checked. It is fixed by the experimenter for the control and the experimental group. Hence, diet is independent variable or the factor being tested.

During the experiment, apart from the independent variable, rest of the factors are kept constant for both the experimental and the control group so that the results are only produced due to the change in the independent variable. Here, age of the mice will be same in both the groups and the initial microbial composition is also same. Hence, age of the mice and initial composition of the microbial community are the factors controlled or kept consistent.

Dependent variable is the factor on basis of which the experiment is analysed. It is the output of the experiment and its value is expected to change with the change in the independent variable. Here, effect of diet is being checked on the mice. Hence change in boy fat, weight gain and final composition of the microbial community are factors to be measured or dependent variables.

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