Definition
Extraneous variables are unwanted influences that may distort the true relationship between the independent and dependent variables. Control refers to techniques used to minimize or neutralize these effects.

Introduction
Even the most careful experiment can be sabotaged by hidden forces—participant moods, environmental changes, or researcher bias. Controlling such variables is essential to isolate the pure effect of the variable being studied.

Explanation
There are several control mechanisms:

Randomization – assigning subjects randomly ensures that uncontrollable factors are evenly distributed across groups.

Matching – pairing participants with similar characteristics like age or income before assigning them to groups.

Statistical Control – using regression or covariance analysis to adjust for unwanted variables.

Environmental Control – maintaining identical settings (temperature, lighting, timing) for all groups.

Blinding – keeping participants or experimenters unaware of group allocations to prevent expectation bias.

For instance, a medical trial testing a new drug must control diet, lifestyle, and concurrent medications to ensure that observed improvements stem solely from the treatment itself.

Key Takeaways
Controlling extraneous variables transforms correlation into causation. It shields findings from bias and enhances the internal validity of the study.

Real-World Case
In pharmaceutical research, double-blind randomized controlled trials are the gold standard precisely because they control human and procedural biases. The placebo group helps determine the genuine effect of the medicine, independent of psychological expectations.
Reference: https://clinicaltrials.gov