The independent variable is the one thing you change on purpose, the dependent variable is what you measure to see the effect, and controlled variables are everything you keep the same so the comparison is fair. In "Does the amount of light change how tall a plant grows?", light is independent, height is dependent, and water, soil and plant type are controlled.
Below are plain definitions, 36 examples across subjects, a quiz with answers, and how variables turn into a hypothesis. If you need a project first, browse science fair project ideas.
What independent, dependent and controlled variables are
A biology textbook from Butte College calls the independent variable the manipulated variable, because the researcher changes it, and the dependent variable the affected or outcome variable. Any other variable that might affect the outcome is held constant.
| Variable | Plain definition | Other names | Question to ask |
|---|---|---|---|
| Independent | What you change on purpose | Manipulated variable, x | What did I change? |
| Dependent | What you measure as the result | Responding or outcome variable, y | What did I measure? |
| Controlled | What you keep the same for every trial | Constants | What did I keep the same? |
How to tell independent and dependent variables apart
Use three quick tests.
Change only one independent variable at a time. If you change light and water together and the plants grow taller, you cannot say which one caused it.
Controlled variables vs the control group
These sound alike but are different.
- Controlled variables are conditions held the same for every group: same pot size, same water, same temperature.
- The control group is the baseline group that does not get the treatment, such as plants with no fertilizer. You compare every other group against it.
A variable you forgot to control, and that changes alongside your independent variable, is a confounding variable. It is the most common reason judges question a result.
Free download: The Science Fair Project Kit
Seven printable worksheets that take a project from first idea to judge-ready: hypothesis builder, experiment design, data log, board layout, and the ISEF scoring rubric with interview prep.
36 independent and dependent variable examples
| # | Experiment | Independent variable | Dependent variable | Controlled variables |
|---|---|---|---|---|
| 1 | Light and plant growth | Hours of light per day | Plant height (cm) | Species, water, soil, pot size |
| 2 | Fertilizer and tomatoes | Fertilizer amount | Number of fruit | Variety, water, light |
| 3 | Seed germination | Soil temperature | Days to sprout | Seed type, water, planting depth |
| 4 | Yeast and sugar | Sugar concentration | Gas produced (mL) | Yeast amount, water temperature, time |
| 5 | Bread mold | Storage temperature | Days until mold appears | Bread type, humidity, container |
| 6 | Handwashing | Washing time (seconds) | Bacterial colonies on a plate | Soap, water temperature, swab method |
| 7 | Enzyme activity | pH of solution | Reaction rate | Enzyme amount, temperature |
| 8 | Earthworm choice | Soil moisture | Worms on each side | Soil type, light, temperature |
| 9 | Dissolving sugar | Water temperature | Time to dissolve | Sugar mass, water volume, stirring |
| 10 | Baking soda and vinegar | Vinegar concentration | Gas volume | Baking soda mass, container |
| 11 | Rusting nails | Salt concentration | Rust mass after 7 days | Nail type, water volume, temperature |
| 12 | Melting ice | Substance added (salt, sugar, sand) | Melting time | Ice cube size, room temperature |
| 13 | Vitamin C in juice | Days in storage | Vitamin C concentration | Juice type, storage temperature |
| 14 | Catalysts | Catalyst type | Oxygen from hydrogen peroxide | Peroxide volume and strength |
| 15 | Ramp and ball | Ramp height | Distance rolled | Ball, surface, release method |
| 16 | Pendulum | String length | Period (seconds) | Mass, release angle |
| 17 | Parachute | Canopy area | Fall time | Drop height, payload mass |
| 18 | Insulation | Insulating material | Temperature drop in 30 minutes | Starting temperature, cup size |
| 19 | Solar panel | Panel angle | Power output (watts) | Light source, distance, panel |
| 20 | Magnet strength | Distance from magnet | Paper clips lifted | Magnet, clip size |
| 21 | Water filter | Filter material | Water clarity after filtering | Water volume, sample source |
| 22 | Acidic water and plants | Water pH | Leaf damage score | Plant type, light, watering schedule |
| 23 | Erosion | Slope angle | Soil lost (g) | Soil type, water volume |
| 24 | Urban heat | Surface type | Surface temperature | Time of day, sun exposure |
| 25 | Algae growth | Phosphate concentration | Algae cell count | Light, water volume, temperature |
| 26 | Music and memory | Background sound | Words recalled | Word list, time allowed, age group |
| 27 | Sleep and reaction time | Hours slept | Reaction time (ms) | Test used, time of day |
| 28 | Screens before bed | Screen time before bed | Minutes to fall asleep | Bedtime, room conditions |
| 29 | Exercise | Minutes of exercise | Heart rate (bpm) | Exercise type, participant |
| 30 | Running shoes | Shoe type | 40 m sprint time | Runner, track, weather |
| 31 | Sorting algorithms | Algorithm used | Run time (ms) | List size, computer, language |
| 32 | Machine learning | Training data size | Test accuracy | Model design, test set |
| 33 | Wi-Fi signal | Distance from router | Download speed | Router, device, time of day |
| 34 | Password strength | Password length | Estimated time to crack | Character set, cracking method |
| 35 | Bridge design | Truss design | Maximum load (g) | Materials, span, glue amount |
| 36 | Battery temperature | Battery temperature | Flashlight run time | Battery type, bulb |
Notice that the independent variable can be a number (hours of light) or a category (shoe type). The dependent variable should always be something you can measure or count.
How variables connect to your hypothesis
A hypothesis is a sentence that predicts how the independent variable will change the dependent variable. Once your variables are named, the hypothesis almost writes itself:
If [independent variable] is [increased / decreased / changed to X],
then [dependent variable] will [increase / decrease / stay the same],
because [reason from your research].
Using row 17: "If the canopy area of a parachute increases, then its fall time will increase, because a larger canopy creates more air resistance." Drop height and payload mass stay controlled.
For 50 more worked sentences in every subject, see the science fair hypothesis examples. When you write up results, the same variables reappear in your abstract, as the science fair abstract examples show.
Variables quiz
Name the independent and dependent variable in each. Answers follow.
Quiz answers
Common mistakes with variables
| Mistake | Fix |
|---|---|
| Changing two things at once | One independent variable per experiment |
| A dependent variable you cannot measure ("plant health") | Pick a number: height, leaf count, mass |
| No control group | Add a baseline group with no treatment |
| Too few trials | Repeat each condition, ideally 3 to 5 times or more |
| Listing "everything" as controlled | Name the specific factors that could affect the result |
| Swapping the axes on the graph | Independent on x, dependent on y |
Judges probe these points in the interview, as the guide on how ISEF is judged explains. A trained researcher catches design flaws before you collect data, which is why students in the YRI Fellowship design their experiment with a PhD mentor before running a single trial.
From science fair project to published paper
Clean variables are what make a project publishable. A paper's methods section is a precise account of the independent variable, the dependent variable, how each was measured and what was controlled, so a project with a clear design is most of the way to a manuscript. What it still needs is a literature review, statistics and a discussion of limitations.
The YRI Fellowship is a 10-week, one-on-one program with a PhD mentor that turns a student's project into a first-author paper submitted to peer-reviewed IEEE, Springer or Elsevier venues, with ISEF and JSHS preparation included. 87% of Fellows publish. To compete first, read how to win science fairs and browse ISEF project ideas by category.
Related guides
- Science fair hypothesis examples
- Science fair abstract examples
- Science fair project ideas
- Science fair poster layout
- How ISEF is judged
Frequently Asked Questions
What is the difference between an independent and dependent variable?
The independent variable is what you change on purpose. The dependent variable is what you measure to see whether that change had an effect.
What is a controlled variable?
A factor you keep the same in every trial, such as temperature or water amount, so that only the independent variable could explain differences in the result.
Can an experiment have more than one dependent variable?
Yes. You can measure plant height and leaf count in the same experiment. Keep one independent variable, though, so you know what caused the change.
Is time an independent variable?
Often, yes. When you measure something every day, time is on the x-axis as the independent variable, and the thing you measure is dependent.
Is the control group a controlled variable?
No. The control group is a baseline group that gets no treatment. Controlled variables are the conditions you keep the same for every group, including the control group.
Which variable goes on the x-axis?
The independent variable goes on the x-axis and the dependent variable on the y-axis.
How do I turn my science fair project into a published paper?
Write up your variables, methods and results as a full manuscript with a literature review and statistics, then submit to a journal that accepts high school authors. The YRI Fellowship pairs you with a PhD mentor for 10 weeks to write a first-author paper for IEEE, Springer or Elsevier venues while you prepare for ISEF or JSHS. Applying takes 10 minutes and has no fee.