A variable refers to something that can change. In an experiment, one thing at a time is changed and the results that change are then measured.   What you want to test – the change you make – is under your control  (what color light to use), what you want to see happen is what you then measure to compare (how tall it grows).

Independent Variable, Dependent Variable, Subjects, Control, and the different Constants

A “variable” is something that can change.
A “constant” is something that does NOT change or is kept the same for all subjects.
The “subjects” are what the test is done to in order to see how they change or react.
A “control” is one subject or group that is left alone and not tested on; it is used as a normal standard for comparison.  

1. Independent Variable: What you want to test. It is the variable you are going to manipulate or change; it is what you are testing.

• I want to know if red is a better color for plants to grow taller.
• I want to see if potting soil is better than my garden soil to make larger flowers.

This is called the Independent variable or the IV. It is what is being tested.  Such as the red color, or potting soil.

Use these sentences:

“I want to know if ________, will ….”   Or   “I want to test ______”

What goes into the blank is the IV.

2. Dependent Variable: What change will happen after you try the new condition (the IV) on the subject?  What will you measure to know if it worked or did not work?  What data will you take?

You will measure resulting changes; these measurements are recorded and are called data.

• I want to know if red is a better color for pants to grow taller.
• I want to see if potting soil is better than my garden soil to make larger flowers.

This is called the Dependent Variable or the  DV. The change in the subject is “dependent” on what is being tested. The plant’s height will be dependent on the color of lights. The size of the flowers is dependent on the potting soil used.

Use this to determine the DV:  What is the data you will be taking? What will you measure?
Use this help you remember:
                   “I want to test___ ”  is the IV    “The Data you will take ____”  is the DV

3. Control: How will you know if the IV you tested worked or not?

If you just try red color and the plant grows 30 cm, did it help or not?
A normal plant grows to 40 cm in the same time, so the red light did not help. Without the normal color on a plant, you would not know if the red color light helps or not. 

If you use a new potting soil and the flowers grow to 15 cm, did the potting soil help or not?
A plant’s flower in normal, regular soil grows to 10 cm, so the new potting soil did help.

The control is the same subject that does not receive the test done on it. It is used as a standard or comparison against the tested subjects.

Use this to help: Thesubject was not tested with the IV, that is the normal, regular one. It will be the same subject (like the plant), but not tested on.  The plant is grown under normal light or in normal, regular soil.

4. Constants: Constant means to stay the same. When conducting a test only one thing can be tested at a time.
Example: If you want to test red lights and you put 1 plant under a red light and one under normal white lights. Then you don’t water plant number 1, but give plant number 2 water.
Plant number one dies. Was it the red lights or lack of water?

To make a test fair, all things must be equal; only the thing you want to test can be different.

Everything must be held constant, but the one thing you are testing. For our examples with plants, the constants would be water, sunlight, temperature, and the same kind/size/age of plants.

Practice what you learned

1. Bailey wants to find out which frozen solid melts the fastest:  water ice, or  juices.  She pours three juices (apple, orange, and cranberry) into the empty cubes of an ice tray, and fills one with pure water and then places the ice tray in the freezer over night.  The next day, she pulls the ice tray out and sets each cube on its own plate.  She then waits and watches for them to melt.  When the last part of the frozen liquid melts, she records the time. She makes sure the cubes are the same size, the plates and air are the same temperature. 

• What would be the hypothesis that Bailey is using?

Based on Bailey’s observations and reading about melting times, she thinks that frozen water (pure ice) will melt faster than water with something else in it. Put that into a short sentence.

Hypothesis:  Pure water ice will melt faster than frozen juice. 

• Independent Variable is what she is testing: I want to find out if frozen juices melt faster than water. The IV is the juices added to water.
• Dependent Variable is what measurement will be taken to determine the results: How fast they melt.  The DV is the melting time.  The time it took to melt.
• Control is the one that is normal, regular, and unchanged. Used as the standard to compare.  The control is pure water ice, which is normal with nothing added.
• Constants are all things that are the same for all groups, what is NOT being tested, and you do not want them to interfere.  If one sample was left to melt in a refrigerator and one sample was put in a warm oven, it would not be a fair test.

Constants: Amount of ice, the temperature of the room, the plate the ice is put on, the humidity and air flow around the ice, the shape of the ice cubes…