What it shows
A hypothetical experiment on fertilizer and plant growth, a common textbook example. Four identical pots get 0, 5, 10 or 15 g of fertilizer, one orange dot per gram.
Each plant’s height is the measured outcome: 14, 18, 22 and 26 cm. The type of plant, type of fertilizer, amount of sunlight and pot size are the same in every pot.
How to read it
Orange is the independent variable: the researcher sets it. Count the dots to read the dose.
The bar at the top of each plant is the dependent variable, read against the height scale in cm.
Identical pots and plants are what “controlled” looks like: those variables do not change between pots.
The experiment
- Independent variable: amount of fertilizer, 0, 5, 10 or 15 g per pot.
- Dependent variable: plant height, 14, 18, 22 and 26 cm.
- The model is height = 14 + 0.8 × fertilizer, so each gram adds 0.8 cm.
- Controlled variables: type of plant, type of fertilizer, amount of sunlight, pot size.
Ways to use it in teaching
Students name the three kinds of variable in a study of their own design, then draw it in the same layout.
Ask what would go wrong if the 15 g pot also sat in the sunniest window.
Pair it with the mediation and moderation diagrams, which extend this same experiment.
For a full explanation of each kind of variable, with more examples, see our guide to independent and dependent variables.
Sources
- Dependent and independent variables. (n.d.). In Wikipedia. Retrieved October 19, 2026, from https://en.wikipedia.org/wiki/Dependent_and_independent_variables
