The scientific method uses experiments to test relationships between different factors, known as variables. Understanding how variables interact is fundamental to establishing cause and effect. A well-designed investigation requires careful management of these variables, especially those that must remain unchanging throughout the testing period.
The Definition of a Controlled Variable
A controlled variable is an element within an experiment that is intentionally held constant or standardized across all test groups. This factor could influence the outcome but is not the specific element being studied by the researcher. Controlled variables are sometimes called constant variables or controlling factors because their value does not change during the investigation. For instance, if testing how different amounts of light affect plant growth, the type of soil used would be a controlled variable.
The purpose of keeping these factors uniform is to ensure that every experimental condition, except for the one being tested, is identical. If a researcher were testing a new medication, they would control the age and health status of the participants in the study to standardize the test groups. The constancy of these variables allows for a clear, apples-to-apples comparison between the different experimental conditions.
Why Controlled Variables Are Essential for Reliable Results
Controlled variables are necessary because they isolate the relationship between the two main factors under investigation. By keeping all other potential influences standardized, scientists can be confident that any observed change in the results is directly attributable to the factor they manipulated. This practice ensures internal validity, which is the confidence that the experimental treatment truly caused the observed effect.
Without controlling these elements, an experiment risks being compromised by confounding factors, which are outside influences that can unintentionally skew the results. For example, if a researcher is testing a fertilizer but fails to control the amount of water each plant receives, they cannot know whether better growth was due to the fertilizer or simply more water. The uncontrolled variable acts as an alternative explanation for the results. Controlling these background conditions eliminates competing explanations, allowing the researcher to accurately conclude the effect of the primary tested factor.
Practical Examples of Controlled Variables in Action
In an investigation determining how different soil pH levels affect the height of tomato plants, multiple factors must be standardized. The independent variable is the soil pH, and the dependent variable is the plant height. The researcher must control the volume of the potting containers, the amount of light exposure, and the ambient temperature of the room. Additionally, using the same species and age of tomato plant seedlings and providing an identical volume of water at the same time each day are necessary controls for this experiment.
Consider a study testing how the concentration of a new cleaning agent affects its ability to remove a specific stain. The independent variable is the concentration of the cleaning solution, and the dependent variable is the percentage of stain removed. To ensure the results reflect only the concentration, the researcher must control the water temperature used for cleaning, the type and size of the stained fabric samples, and the exact scrubbing technique applied to each sample. The duration the cleaning agent soaks into the fabric must also be identical across all test groups.
A third scenario involves testing whether different colors of light affect a person’s reaction time to a visual stimulus. The independent variable is the color of the light, and the dependent variable is the measured reaction time in milliseconds. Controlled variables include the distance between the participant and the stimulus screen, the brightness of the light, and the ambient noise level in the testing environment. The time of day the test is administered and the specific instructions given to each participant must also be standardized to prevent those factors from influencing reaction speed.
Comparing Controlled, Independent, and Dependent Variables
Every scientific experiment involves a minimum of three categories of variables, each with a distinct functional role. The independent variable is the factor the experimenter actively changes or manipulates between test groups to observe an effect, representing the proposed cause.
The dependent variable is the factor that is measured or observed as a result of the changes made to the independent variable. It represents the outcome or the effect in the experiment, and its value is hypothesized to depend on the independent variable. Controlled variables, in contrast, are the factors that are deliberately held constant throughout the entire experiment. They are not the focus of the study, but their unchanging state prevents them from interfering with the specific relationship being tested between the independent and dependent variables.