Controlled Experiment – Meaning, Methods & Examples

01.10.22 Experiments Time to read: 5min

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In a controlled experiment, the researcher manipulates one variable, the independent variable, while keeping all other variables constant to observe the effect on the dependent variable. This careful control forms the core of this methodology, eliminating the possibility of external factors influencing the outcome and thus allowing the researcher to draw stronger conclusions about cause-and-effect relationships.

Controlled Experiment – In a Nutshell

  • Controlled experiments allow researchers to measure the effect of particular variables.
  • A controlled experiment is used to divide population samples into control and experimental groups to estimate the effect of a critical variable.
  • Control experiments can be carried out in different ways, including a disguised approach where participants are unaware of the investigated variable.

Definition: Controlled experiment

Experiments in research entail manipulating or changing an independent variable and assessing its effect on the dependent variable or outcome. All the variables except the independent variable are held constant in a controlled experiment. Researchers begin by defining the variables present to ensure objective measurement of the essential variables.

The variables held constant are known as control variables. Researchers can control variables by:

  • Maintaining variables at a regulated level
  • Analyzing variables for statistical purposes
  • Using random tests in an experiment
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Why is a controlled experiment important?

A controlled experiment ensures internal validity, essential for establishing cause and effect through experimentation.


You want to study the effects of music on productivity. You want to see if music improves people’s productivity.

  • Your independent variable is music
  • Your dependent variable is the hours that music listeners stay productive

Many factors can influence productivity. A controlled experiment is ideal to test whether listening to music affects productivity.

Extraneous variables – These are factors that you don’t measure but which may affect the dependent variable. You should eliminate the effects of these variables to create strong internal validity. Extraneous variables in this example include:

  • The type of work
  • The workplace environment
  • The worker’s level of skill
  • The worker’s level of income

In an uncontrolled experiment, these variables may affect a worker’s productivity, which makes it hard to assess the true impact of music on productivity.


Methods used in controlled experiments

Researchers use standard data collection practices to control variables in a study. The same tools should be used in a shared environment to test all the participants. A systematic approach is applied to change only the independent variable, for instance, the type of music.

Sampling methods can be used to manage other extraneous variables. You should select the most representative sample based on a criterion that considers all the essential characteristics, for example, using participants from a specific job description rather than short participants. Noting the extraneous variables in your study can help in further analysis.

Once the participants have been identified, they are grouped to test individual independent variables. The identifiable groups guide the implementation of controls in the experiment.

Control groups

A controlled experiment uses control groups. Researchers use control groups to test the effect of a given variable by comparing the control group with an experimental group. The control group is isolated from the experimental group to ensure the independent variable does not affect the control group.


In the music experiment, the participants can be placed in identical rooms for the duration of the study. The participants can be placed in one of two groups to determine the effect of music on productivity:

  • A control group listening to jazz music and doing routine tasks
  • An experimental group listening to classical music doing the same everyday tasks

The only difference between these groups is the type of music. All other characteristics are identical.

Random assignment

A random assignment prevents systematic differences between an experimental group and the control group. It involves placing the research subjects into random groups. Every participant has an equal opportunity to be placed in the experimental group.

Randomization is necessary for “true experiments”, as opposed to quasi-experiments, where the subjects are chosen based on a criterion.


The workers in our previous example can be assigned random numbers. Odd and even numbers can be grouped together to form experimental and control groups.

Random assignment ensures the two groups can be compared based on age, role, gender, and other observable traits. This enables comparison between the groups.

Masking (blinding)

Masking in a controlled experiment involves hiding some variables from the researchers and the subjects. Researchers often intentionally conceal the presence of certain variables in a study to prevent bias that may arise due to the participants’ awareness. Masking ensures strong internal validity by ensuring the participants behave naturally.

Researchers may unknowingly influence the behavior of participants. Other environmental cues may also nudge participants to behave in a particular way. Masking guarantees that no undue biases can affect the result of the study.


For the music study, a researcher can introduce other cues such as art or snack in the workplace. By disguising the experiment’s true purpose, the participants are unaware of the variable that the researcher is interested in.

Problems with controlled experiments

Controlled experiments may be an effective research tool. However, there are some challenges associated with its methodology

Difficulties controlling all variables

Every research participant behaves in a certain way based on their beliefs, personality, attitudes, and other factors. This poses a problem for researchers attempting to hold the extraneous variables constant because there are too many variables. However, a researcher may limit the extraneous variables to manage their influence on the outcome.

Risk of low external validity

Many controlled experiments do not apply to other contexts outside the experiment. A highly controlled experiment may lack real-world relevance as it considers a limited list of variables. Researchers need to consider the objectives of their experimental studies to prioritize control or applicability.

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A control experiment is a method of experimental research where only the independent variable is changed. The dependent variables are held constant in a controlled environment to isolate the effect of one variable.

The control group in a controlled experiment is the group that doesn’t receive a variable. Experimental programs such as clinical trials use control groups in testing new drugs by comparing those who receive treatment and those who don’t.

Sometimes, researchers may be concerned about the effect of a subject’s awareness on the findings of a research project. Masking is a deliberate action by researchers to hide some variables in a study as a measure to build strong internal validity.

Controlled experiments are commonly used in healthcare research. New treatment methods are tested on groups of patients to test the efficacy and other variables that may affect their effectiveness.