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 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.

Identify the components of this experiment by matching the terms with the appropriate category below:

Factors being tested Factors controlled Factors to be measured
(independent variable) (Kept consistent) (dependent variable)

1. initial composition of the microbial community
2. age of the mice
3. change in body fat
4. diet
5. weight gain
6. final composition of the microbial community

The answers are:

1. initial composition of microbial community (controlled variable)

2. age of the mice (controlled variable)

3. change in body fat (dependent variable)

4. diet (independent variable)

5. weight gain (dependent variable)

6. final composition of the microbial community (controlled variable)

Explanation:

Before I go ahead to explain each of the choices of classification, let us define each of the different types of variable.

1. Independent variable: this is a variable that is under the control of the experimenter. It is made up of the factors that are being tested. In our example, the independent variables is; diet. The aim of the experiment is to study the effect of the type of diet on the mice. The diet can also be varied and assigned at will by the experiment.

2. Dependent variable: a dependent variable is one whose outcome is reliant on the effect of the changes in the independent variable. It is the factor to be measured. In our example, the factors that obey these definitions are; change in body fat and weight gain. these two factors depend on the types of diet taken by the mice, hence they are the factors to be measured.

3. Controlled variable: these are variables that are kept constant throughout the experiment to rule out biases, because they produce the same effect on the dependent variable as the independent variable and must be keep constant or uniform in the test groups. in our example, the factors that are controlled include; initial and final compositions of the microbial communities and the age of the mice, because the microbial composition if not uniform among the groups can cause  biases in how the diets are processed, and the age of the mice also must be uniform to avoid age-related differences in the processing of the diets

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