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What are Variables? How to use them in Your Science Projects

What are Variables?

What are Variables?

In science, a variable is any factor, trait, or condition that can exist in differing amounts or types.

Scientists try to figure out how the natural world works.To do this they use experiments to search for cause and effect relationships. Cause and effect relationships explain why things happen and allow you to reliably predict the outcomes of an action. Scientists use the scientific method to design an experiment so that they can observe or measure if changes to one thing cause something else to vary in a repeatable way.

These factors that change in a scientific experiment are variables.

A properly designed experiment usually has three kinds of variables: independent, dependent, and controlled.

What is an Independent Variable?

The independent variable is the one that is changed by the scientist. Why just one? Well, if you changed more than one variable it would be hard to figure out which change is causing what you observe. For example, what if our scientific question was: "How does the size of a dog affect how much food it eats?"; then, during your feeding experiments you changed both the size of the dog and the time of day the dogs were fed. The data might get a bit confusing— did the larger dog eat less food than the smaller dog because of his size or because it was the middle of the day and dogs prefer to eat more in the morning?

Sometimes it is impossible to just change one variable, and in those cases, scientists rely on more-complicated mathematical analysis and additional experiments to try to figure out what is going on. Older students are invited to read more about that in our Experimental Design for Advanced Science Projects page. To be clear though, for a science fair, it is usually wise to have only one independent variable at a time. If you are new to doing science projects and want to know the effect of changing multiple variables, do multiple tests where you focus on one independent variable at a time.

What is a Dependent Variable?

The dependent variables are the things that the scientist focuses his or her observations on to see how they respond to the change made to the independent variable. In our dog example, the dependent variable is how much the dogs eat. This is what we are observing and measuring. It is called the "dependent" variable because we are trying to figure out whether its value depends on the value of the independent variable. If there is a direct link between the two types of variables (independent and dependent) then you may be uncovering a cause and effect relationship. The number of dependent variables in an experiment varies, but there can be more than one.

What is a Control Variable?

Experiments also have controlled variables. Controlled variables are quantities that a scientist wants to remain constant, and she or he must observe them as carefully as the dependent variables. For example, in the dog experiment example, you would need to control how hungry the dogs are at the start of the experiment, the type of food you are feeding them, and whether the food was a type that they liked. Why? If you did not, then other explanations could be given for differences you observe in how much they eat. For instance, maybe the little dog eats more because it is hungrier that day, maybe the big dog does not like the dog food offered, or maybe all dogs will eat more wet dog food than dry dog food. So, you should keep all the other variables the same (you control them) so that you can see only the effect of the one variable (the independent variable) that you are trying to test. Similar to our example, most experiments have more than one controlled variable. Some people refer to controlled variables as "constant variables."

In the best experiments, the scientist must be able to measure the values for each variable. Weight or mass is an example of a variable that is very easy to measure. However, imagine trying to do an experiment where one of the variables is love. There is no such thing as a "love-meter." You might have a belief that someone is in love, but you cannot really be sure, and you would probably have friends that do not agree with you. So, love is not measurable in a scientific sense; therefore, it would be a poor variable to use in an experiment.


Examples of Variables

Question Independent Variable
(What I change)
Dependent Variables
(What I observe)
Controlled Variables
(What I keep the same)
How much water flows through a faucet at different openings? Water faucet opening (closed, half open, fully open) Amount of water flowing, measured in liters per minute
  • The faucet
  • Water pressure, or how much the water is "pushing"

"Different water pressure might also cause different amounts of water to flow and different faucets may behave differently, so to ensure a fair test, I want to keep the water pressure and the faucet the same for each faucet opening that I test."

Does heating water allow it to dissolve more sugar? Temperature of the water measured in degrees Celsius Amount of sugar that dissolves completely, measured in grams
  • Stirring
  • Type of sugar

"More stirring might also increase the amount of sugar that dissolves, and different sugars might dissolve in different amounts, so to ensure a fair test I want to keep these variables the same for each cup of water."

Does fertilizer make a plant grow bigger? Amount of fertilizer, measured in grams
  • Growth of the plant, measured by its height
  • Growth of the plant, measured by the number of leaves
  • See Measuring Plant Growth for more ways to measure plant growth.
  • Same type of fertilizer
  • Same pot size for each plant
  • Same plant type in each pot
  • Same type and amount of soil in each pot
  • Same amount of water and light
  • Make measurements of growth for each plant at the same time

"The many variables above can each change how fast a plant grows, so to ensure a fair test of the fertilizer, each of them must be kept the same for every pot."

Does an electric motor turn faster if you increase the voltage? Voltage of the electricity, measured in volts Speed of rotation, measured in revolutions per minute (RPMs)
  • Same motor for every test
  • The motor should be doing the same work for each test (turning the same wheel, propeller, or whatever)

"The work that a motor performs has a big impact on its speed, so to ensure a fair test, I must keep that variable the same."

Time as an Example of an Independent Variable

In some experiments, time is what causes the dependent variable to change. The scientist simply starts the process, then observes and records data at regular intervals.

Question Independent Variable
(What I change)
Dependent Variables
(What I observe)
Controlled Variables
(What I keep the same)
How fast does a candle burn? Time measured, in minutes Height of candle, measured in centimeters, at regular intervals of time (for example, every 5 minutes)
  • Use same type of candle for every test
  • Wind—make sure there is none

The Independent Variable for Surveys and Tests of Different Groups

When a scientist performs a test or survey on different groups of people or things, those groups define the independent variable. For example:

Question Independent Variable
(What I change)
Dependent Variables
(What I observe)
Controlled Variables
(What I keep the same)
Who listens to music the most: Teenagers or their parents? Groups receiving the survey: Teenagers or parents Amount of time that each person listens to music per day, measured in hours Ask the question in exactly the same way to each individual

Either/Or (Binary) Variables

Sometimes a variable simply represents an either/or (binary) condition. For example, something might be either present or not present during an experiment.

Question Independent Variable
(What I change)
Dependent Variables
(What I observe)
Controlled Variables
(What I keep the same)
Is a classroom noisier when the teacher leaves the room? Teacher location: The teacher is either in the room or not in the room.

"The teacher's location is an either/or situation"
Loudness, measured in decibels
  • Same classroom
  • Same students
  • Same time of day
Do bicycle fenders keep the rider dry when riding through a puddle? Fenders: The bicycle either has fenders or it does not

"Many engineering projects have alternative designs with independent variables like this one (with and without fenders)."
The rider either gets wet or does not.

"Dependent variables can represent either/or situations, too."
  • Same type of bike and tires (except for the fenders!)
  • Riding at the same speed
  • Same size and depth of puddle



Here is a sample containing the variables and hypothesis.


Science Fair Project Variables Checklist

What Makes for Good Variables? For Good Variables, You Should Answer "Yes" to Every Question
Is the independent variable measurable? Yes / No
Can you change the independent variable during the experiment? Yes / No
Have you identified all relevant dependent variables, and are they all caused by and dependent on the independent variable? Yes / No
Are all dependent variable(s) measurable? Yes / No
Have you identified all relevant controlled variables? Yes / No
Can all controlled variables be held at a steady value during the experiment? Yes / No
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