Independent vs dependent
01 // Two variables, one job each
Every fair test has one variable the experimenter deliberately changes, and one variable the experimenter measures to see if it responded. The independent variable is the one you change on purpose, chosen before the experiment starts. The dependent variable is the one you measure — its value depends on what the independent variable did.
A quick way to remember it: the independent variable goes on the x-axis of a graph, the dependent variable goes on the y-axis. If you are not sure which is which in a word problem, ask "what did the experimenter choose to set up?" — that one is independent.
02 // Independent vs dependent
Independent
What you change- Chosen before the test
- Plotted on the x-axis
- One per experiment
Dependent
What you measure- Recorded in every trial
- Plotted on the y-axis
- Always has a unit
03 // Worked example
A student waters three identical tomato plants with 0, 50, or 100 mL of fertilizer solution each week and measures fruit count after 8 weeks.
| What was chosen before the test | amount of fertilizer (0, 50, 100 mL) |
| What was measured after the test | number of tomatoes per plant |
| Kept the same in every trial | pot size, soil, water, sunlight, plant variety |
Formula Independent = what changes between groups. Dependent = what you measure in response.
04 // Common mix-ups
- Time is almost always independent when an experiment tracks change over days or weeks — the days pass regardless of the result.
- A variable can be dependent in one experiment and independent in another. Temperature is dependent in a reaction-rate test if you're measuring it, but independent if you're the one setting it.
- "Controlled variable" is a third category, not a type of independent or dependent variable — see controlled variables.
05 // Questions people ask
Which axis does the independent variable go on?
The independent variable goes on the horizontal x-axis, and the dependent variable goes on the vertical y-axis, because the dependent variable's value depends on the independent variable's value.
Can an experiment have more than one independent variable?
A basic fair test changes exactly one independent variable at a time. Advanced experiments can test two or more together, but that requires a larger design (like a factorial experiment) so the effects of each variable can still be told apart.
Is time always the independent variable?
Usually, in experiments that track change over time, but not always — time is only the independent variable if the experimenter is using it to organize when measurements happen, rather than measuring it as a result.