The independent variable is the one thing deliberately changed.
Independent variable The single factor deliberately changed in an experiment.
Step 1: Something to Notice
Watch first. The explanation comes later.
Two trials, timed to the second, with a clear difference between them — and no way to say what caused it.
How can careful data support nothing?
Step 2: Find Out
Isolate the number of variables changed and watch the conclusion disappear.
Step 1 — Predict
Two variables change between trials. What can the result establish?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
The dependent variable is what you measure
The outcome that may or may not respond to the change.
Dependent variable The outcome measured in response to the change.
Everything else is held fixed
These are the controlled variables, and forgetting one is what confounds a design.
Confounding destroys attribution
Two changes at once, and the result could belong to either. The data cannot be repaired.
And a control group shows the baseline
The trial where the independent variable is not applied, so the rest of the procedure is visible.
A precise result can still support nothing
If two things changed at once, no amount of measurement precision identifies which caused the effect. Controlling variables is what makes a result interpretable, and it is prior to accuracy.
Repeat, then look at the spread
Three runs that agree closely support the result; three that scatter widely mean something uncontrolled is at work. The spread is data about the experiment itself.
Step 4: A Common Mistake
Lots of people think
“If the measurements are careful, the experiment is good.”
Step 5: Why Drug Trials Hide Which Pill Is Which
The same idea somewhere new.
A patient who knows they received the real drug reports differently, and so does a doctor who knows which arm a patient is in. Double-blind design removes both, holding expectation constant between the groups — a controlled variable like any other, just an unusually hard one to hold. The placebo group is the control, and the difference between the two arms is the only quantity the trial is actually measuring.
Step 6: Think It Through
Practice makes it stick.
Two Changes
Problem 1 of 2
Both the temperature and the particle size differ between trials. What can you conclude?
The Blinded Trial
Problem 2 of 2
Why do drug trials hide which pill is which?
Designing It Properly
1 of 5
How many independent variables should an experiment have?
2 of 5
Can careful measurement rescue a confounded design?
3 of 5
What is a control group?
4 of 5
What is the dependent variable?
5 of 5
Why is a confounded result dangerous rather than merely useless?
Step 7: Quick Check
Show what you know.
Question 1 of 1
Careful measurements were taken and two variables changed. Is the experiment good?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
Explain what makes an experiment controlled, and why a confounded design cannot be rescued by careful measurement.
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- A controlled experiment differs in exactly one respect.
- The independent variable is changed; the dependent variable is measured.
- Confounding destroys attribution, and no later care repairs it.
- A control group shows what the rest of the procedure does on its own.