Biology Honors 12
Part II. Scientific Method: Controlled Variables
There are many different types of experimental procedures that can present the experimenter with problems that must be solved before reliable data can be collected. One such problem presents itself when many variables affect the dependent variable. For example, you want to determine the best method for studying for a test. You have taken many tests and have an idea that making note cards before a test will help you get a good grade. You pose a question, “Will using note cards help me get a good grade on tests?” You set up a test in which you use note cards for one test (experimental group) and plan to take the next test without note cards (control group). Your independent and experimental variable will be the note cards, the dependent variable will be the grade you get on the test.
Immediately, you see the problems. What if one test is a math test (hard) and the other is a health test (nobrainer)? What if you have time to study for one test and don‟t have the time to study for the other test? What if you are feeling great the day you take one test and feel lousy the day you take the next test? All of these differences are variables that can affect the dependent variable (your grade). You need a way to control these variables so that you can be sure that any differences between the data in the control group and the experimental group is caused by the experimental variable alone.
Controlled variable -variables that can affect the dependent variable. Controlled variables are „controlled‟ by the experimenter. They are held same between both the control group and the experimental group so that the only variable that is different between the two groups is the experimental variable. Once you recognize what variables you need to control, you can set up your experiment. You use note cards for two different history tests. You plan to study 2 hours for each test. You eat breakfast before each test and only use the data from a test if you feel good. You are reducing the variables that may interfere with the reliability of your results. The only variable remaining is the use of note cards during one test. That is your experimental variable.
Independent and Dependent Variables
The graphs you just looked at are drawn using specific methods. For example, in the body mass vs. heart mass graph, the body mass is on the X, or horizontal axis and the heart mass is on the Y or vertical axis. Why did the experimenter do this? Couldn‟t the body mass have be graphed on the vertical axis and the heart mass on the horizontal axis? The answer lies in the identification of two different variables: Independent Variable- Independent variables may affect the data or the outcome of the experiment. In all experiments, the experimental variable is an independent variable. It is the factor in the experiment that the experimenter is most interested in. If there is only one independent variable (the experimental variable), it is graphed on the X-axis. If an experiment has an experimental variable and an additional independent variable, such as time, time would be graphed on the X axis, and the experimental variable is drawn as different lines on the graph. Keep in mind that in some experiments, time will be the experimental variable. Dependent Variable- The „d‟ in dependent stands for „data.‟ Don‟t be confused by the name „variable.‟ The dependent variable is simply the data collected during the experiment. The dependent variable is always graphed on the Y axis.
Knowing this, you can understand why the experimenter graphed the body/heart data the way he did. The prediction statement indicated that the size of the heart was dependent on the size of the organism, not that the size of the body was dependent on the size of the heart! The heart mass is therefore the dependent variable and the body mass is the independent variable.
The second graph with the oxygen uptake by lungs and...
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