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A controlled variable , or control variable, is a variable that is kept constant or under control during the execution of an experiment, in such a way that its variation does not invalidate the results of the experiment . The controlled variables are not part of the dependent variables or the independent variables in an experiment. However, it is generally an intervening variable that can have significant effects on the dependent variable, but whose effects on it are not of interest in the experiment.
For example, if in an experiment you want to study the effect of exercise on muscle mass, in which muscle mass is the dependent variable and the amount of exercise is the independent variable, it is logical to assume that there are other variables that could also affect the results thereof. For example, diet also plays a vital role in the production of muscle mass since, if we do not eat protein, the body will have no raw material with which to produce muscle mass, regardless of the amount of exercise the individual does. For this reason, every effort would be made to ensure that all subjects participating in the experiment have a similar or equivalent diet.
Importance of the controlled variable
Controlled variables are important for the following reasons:
Improves the reproducibility of an experiment
Controlling as many intervening variables as possible facilitates the reproduction of an experiment, either by the same researcher or by another who wishes to verify the results of the first. This is because this makes it possible to more accurately describe the exact conditions under which an experiment was carried out, ensuring that when it is repeated, the same results can be obtained.
Provides validity to the results of an experiment
Specifying which variables were controlled during an experiment allows other researchers to understand what exactly was done. This gives more validity to the results as it minimizes any reasonable doubt about which variable is really responsible for the changes in the dependent variable.
Failure to control them can invalidate the results.
The opposite of the previous section is also true. Controlling the intervening variables validates the results, but not controlling them can also invalidate them. This is mainly because the question arises as to whether the change in the dependent variable is due to the independent variable or to another unknown “confounding variable” that is affecting the results. For example, in a biochemical experiment in which the activity of an enzyme against a substrate is being studied, not fixing the pH using a buffer or monitoring it makes the results totally unreliable and, therefore, completely useless, since that the enzymatic activity strongly depends on the pH, and this can vary as a consequence of the same chemical reaction.
Measurement and control of the controlled variable
As can be deduced from the previous example, controlled variables are of great importance in any experiment, since they have the power to invalidate the results, no matter how carefully the rest of the experimental procedure has been carried out. These variables are usually measured and reported as part of the general conditions in which an experiment was carried out.
Some of these variables are literally under the control of the experimenter, who can set them to a desired value and keep them constant during the experiment. An example is the temperature in a chemical reaction that takes place in a thermostated bath or the temperature inside a fermentation chamber.
In other cases, the researcher cannot do anything to change the value of the controlled variable, and can only measure it and report it as part of the experimental data. Such is the case of atmospheric pressure, the intensity of sunlight, humidity, the amount of rain, and any other intervening variable that is out of your control.
Examples of controlled variables
Temperature
Temperature is a variable that affects virtually any scientific experiment. Both in physics, in chemistry and in biology, as in any other related discipline, temperature is one of the main controlled variables, when it is not the independent variable. This variable is measured with the use of thermometers and is controlled and manipulated using thermostats.
The pressure
Pressure is another of the important controlled variables in many types of experiments. Its effects on solid or liquid systems are not usually so marked, but they are when it comes to gaseous systems, since these are compressible and their volume can vary considerably with pressure. In some experiments carried out in closed chambers, the pressure can be controlled at will, but when the experiment is exposed to air, then it is subject to variations in atmospheric pressure. In these cases, all the investigator can do is measure and record barometric pressure on a regular basis.
pH value
pH can affect many chemical reactions and practically all biochemical reactions, which is why it is, together with temperature, one of the most frequently controlled variables in these kinds of experiments. This is achieved by using pH-meters to measure the pH and buffer solutions to set it to a predetermined value.
ionic strength of a solution
Ionic strength is a measure of the total concentration of ions present in a solution. Although these ions may not be directly involved in a chemical or biochemical reaction, they often affect the activities of other ions that are involved. For this reason, ionic strength is a controlled variable.
Age
In many experiments in medicine, but also in the social sciences, age is a very important factor that can affect the results. This is why many of these experiments are designed in such a way that they only work with people of a certain age or with a certain range of ages, despite the fact that age is not the variable of interest.
The level of education
Just as age can affect the results of a socioeconomic study, so can the level of education. To prevent this variable from invalidating the results, it is usually controlled.
References
Bhandari, P. (2021, April 19). Control variables explained .
Chang, R., & Goldsby, K. (2015). Chemistry (12th ed.). New York, New York: McGraw-Hill Education.
https://www.scribbr.es/uncategorized-es/marco-conceptual-variables-de-control/
