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 Design
Constants
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 Design
Independent
Variable
?
Dependent
Variable
?
Constants
?
Terry’s Experimental Design
Independent
Variable
Kind of seed:
Expensive Cheap
Dependent
Variable
Number (and perhaps kinds) of birds
that visit
Constants

Location of feeders
Type of feeder
Way 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?