Variables in Science Experiments

What makes an experiment “fair”?

Variables

A variable is something that is changed A constant, or control, is something that does

not change

Science experiments use…

Independent Variable: the one factor that is changed by the person doing the experiment

Dependent Variable: the factor which is measured in the experiment

Constants: all the factors that stay the same in an experiment

A birdy example…

Imagine you want to see what color of bird feeders your local birds preferred.

Red? Blue? Green?

Our Constants and Variables

Independent Variable: color of the feeders Dependent Variable: amount of seed eaten Constants: everything else that is kept the same,

for example:– the location of the feeders– the kind of feeder used– putting the feeders out at the same time

Our Experimental DesignConstants Location of feeders

Kind of seed

Type of feeder

Independent

Variable Red Blue Green

Dependent

Variable

Amount of Seed Eaten

Amount of Seed Eaten

Amount of Seed Eaten

If everything except the independent variable is held constant, we can

say:

The experiment is FAIR.

(ONLY the independent variable can change!)

Why is it important to only change the independent variable?

If you don’t…

If you measure a change in the dependent variable, you won’t know whether it is the independent variable that is causing the change.

Independent Variable

Is Sam’s experiment fair?

YES? NO?

NO! It isn’t fair since the locations of the feeders were very different!

Is Maria’s experiment fair?

YES? NO?

YES! It seems fair since Maria only changed the feeder color!

Is Jessie’s experiment fair?

YES? NO?

NO! It isn’t fair since the feeders are not out at the same time, and we know that the weather was quite different each time. Perhaps the number and kind of birds that visited during the weeks were different due to the weather.

What do you think?Terry loves birds and wants to see as many as

she can in her yard. She wants to know whether she is wasting money buying an expensive mixed seed, when a cheaper brand just arrived at her local store. She wonders… will the expensive seed attract more birds to her back yard?

How would you advise her to find out if the expensive seed is better?

Terry’s Experimental DesignIndependent

Variable ?

Dependent

Variable ?

Constants ?

Terry’s Experimental DesignIndependent

Variable

Kind of seed:

Expensive Cheap

Dependent

Variable

Number (and perhaps kinds) of birds that visit

Constants Location of feedersType of feederWay she measures the amount of seed eaten and the way she counts birds

$

What experiments are you planning?

• What are your variables?• How will you keep your experiment fair?