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Identifying Variables

Three types of tomatoes

Three types of tomatoes (MOs810, Wikimedia Commons)

Learn how scientists define independent, dependent and controlled variables in experimental inquiry.

Identifying Variables

As was mentioned in the Asking Testable Questions backgrounder, testable questions define the variables. In other words, what is being changed and what is to be kept constant, in an experimental inquiry.

What are variables in an experimental inquiry?

Scientists often use experimental inquiries to observe cause and effect relationships. In order to do so, scientists aim to make one change (the cause or ) in order to determine if the variable is causing what is observed (the effect or ).

An experimental inquiry typically has three main types of variables: an independent variable, a dependent variable and controlled variables. We will look at each of these three types of variables and how they are related to experimental inquiries involving plants.

Independent Variables

The independent variable, also known as the, is the difference or change in the experimental conditions that is chosen by the scientist (the cause). To ensure a fair test, a good experimental inquiry only has one independent variable and that variable should be something that can be measured quantitatively. For example, experimental inquiries about plants may include such independent variables as:

  • Volume of water given to plants
  • Nitrogen or phosphorus concentration in soil
  • Duration, intensity or wavelength of light plants are exposed to
  • Concentration or type of fertilizer

Dependent Variables

When a scientist chooses an independent variable (the cause), that person anticipates a certain response (the effect). This response is known as the dependent variable. The dependent variable should be something that is observable and measurable. Like the independent variable, an experimental inquiry should only have one dependent variable. For example, experimental inquiries about plants may include such dependent variables as:

  • Days to germination
  • Surface area of leaves
  • Days to flowering or fruiting
  • Dry mass (amount of plant material after all water has been removed)

Testable Question

How does the volume of water affect the number of days it takes for a tomato plant to flower?

Relationship between an independent and a dependent variable
Relationship between an independent and a dependent variable (©2022 Let’s Talk Science).
Image - Text Version

Shown is a colour illustration explaining the relationship between an independent and a dependent variable. 

On the left is a blue oval with the word "Cause" inside it. This is labelled "Independent Variable" at the top, and "E.g., volume of water" below. On the right is a green rectangle with the word "Effect" inside. This is labelled "Dependent Variable" at the top, and "E.g., days to flowering" below. A red arrow points from cause on the left to the effect on the right.

Controlled Variables

In order for a scientist to ensure that only the independent variable is affecting the dependent variable, all the other factors acting upon the test situation (or test subjects) must be kept constant. The factors that must be kept the same are called the , or constant variables. In a given inquiry, there may be one or more variables that will need to be kept constant. For example, for an experimental inquiry in which you are interested in how the volume of water (independent variable) affects the days to flowering (dependent variable), you would want to keep constant:

  • The type of seeds
  • The type of soil
  • The light source
  • The humidity in the room
  • The type of container (e.g., plastic pots vs. clay pots)
  • The Temperature
Tomato plants in a greenhouse
Tomato plants in a greenhouse (Source: Goldlocki [GNU-FDL] via Wikimedia Commons).
Image - Text Version

Shown is a colour photograph of tomato plants in a greenhouse. 

Rows of tomato plants on both sides of the photograph stretch into the distance. Light comes in through a translucent ceiling. The plants are thick with green leaves. Tomato fruit is visible at the bottom of each plant. Most of the fruit is red and some is green.

A failure to control variables other than the independent variable will mean that you will not know which factor is actually causing the effects you see. In the example above, if some of the plants were sitting closer to the window than others, the differential exposure to light could be affecting the number of days to flowering, rather than the volume of water.

For more about designing experiments, see: Setting Up a Fair Test

What are the variables in Tomatosphere™?

In the Seed Investigation, students investigate the germination rates of tomato seeds that have been to space (or exposed to space-like conditions) with seeds that have remained on Earth.

The testable question in the Seed Investigation is:


Independent variable: type of seeds used - Earth seeds versus space seeds (sometimes seeds are treated to space-like conditions in years when seeds do not go to space)

Dependent variable: number of seeds that germinate

Guided Practice

Exercise 1

Have students read the following questions and determine the independent, dependent and potential controlled variables.

  1. How does the duration of light exposure affect the surface area of tomato plant leaves?
  2. How does the concentration of nitrogen fertilizer affect the days to flowering of tomato plants?
  3. How does the volume of water (mL) affect the number of days to germination of tomato plants?

Exercise 2

In their own words, have students define the terms “Independent variable,” “Dependent variable,” and “Controlled variable.”

Exercise 3

Have students brainstorm the variables that should be controlled in the Seed Investigation (e.g., quantity of water, type of soil, type of planting container, temperature, etc.).

Exercise 4

Have the students think about the Seed Investigation and brainstorm variables that may not be controllable (e.g., giving plants different amounts of water, some plants being closer to a heat vent than others, using different types of soil, etc.).

  1. Independent variable: duration of light (hours)
    Dependent variable: surface area of plant leaves (Overall? Largest leaf? All leaves?)
    Controlled variable(s): quantity of water, type of soil, depth of seeds, source of light, concentration/type of fertilizer (if any); temperature of the room, etc.
  2. Independent variable: Concentration of nitrogen fertilizer
    Dependent variable: days to flowering (when first flower on plants open)
    Controlled variable(s): Same type of seeds, same quantity of water, same type of soil, same source of light, same duration of light, etc.
  3. Independent variable: Volume of water in ml (per day)
    Dependent variable: days to germination (when first seed germinates)
    Controlled variable(s): Single type of seeds, same type of soil, same volume of soil, same type of pots, same source of light, same duration of light, temperature of the room, same time of day for watering, etc.


What are variables? How to use them in your science projects
This page from Science Buddies explains different sorts of variables and how to use them to answer sample questions.

Controlled Variables
This article by Explorable covers variables, control groups, and the value of consistency.

What are Independent and Dependent Variables? (2019)
This article by ThoughtCo explains how to tell the difference between independent and dependent variables, and how to plot variables on a graph.

Identifying and Controlling Variables in Scientific Investigations (2015)
This video (3:16 min.) from SciExperiment Basics explains how to identify and control variables in a scientific inquiry.