Variables in Science
Experiments
What makes an experiment “fair”?
Science experiments use…
Independent Variable: the one and
only factor that is changed in an
experiment.
Dependent Variable: the factor which
is measured/observed in the
experiment. Responds to changes in
the independent variable.
Controlled variables: 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?
Variables
Independent Variable: color of the feeders
Dependent Variable: amount of seed
eaten
Controlled: 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
Controlled
variables
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!)
Is Sam’s experiment fair?
YES? NO?
Why is it important to change
only 1 thing at a time?
If you don’t…
then you won’t know what is causing
your results.
Independent Variable
How does diet affect a fish’s
ability to swim through a
maze?
Patrick thinks fish that each microwaved
food are smarter than fish that do not.
Patrick times the fish going through a
maze before and after eating.
10 fish get microwaved food.
10 fish get non-microwaved food.
Variables
Independent: Microwaved food or not
Dependent: time it takes to get through
the maze
Controlled: Fish species, maze, amount
of food, type of food.
How does a special juice
affect worker productivity?
Smithers thinks that a special juice will
increase the productivity of workers. He
creates two groups of 50 workers each and
assigns each group the same task (in this
case, they're supposed to staple a set of
papers). Group A is given the special juice
to drink while they work. Group B is not
given the special juice. After an hour,
Smithers counts how many stacks of
papers each group has made. Group A made
1,587 stacks, Group B made 2,113 stacks.
Smither’s experiment
Independent variable: Juice or not
Dependent variable: Number of stacks
of papers
Controlled variables: 50 workers were in
each group. They shared the same task
Uncontrolled: The group that did not
consume juice should have consumed
the same amount of water.
How does diet affect the
length and complexity of
starling bird songs?
Contaminated versus Noncontminated food source.
Starlings that ate a controlled diet were
compared to those that ate earthworms from
a sewage treatment plant.
The starlings that ate earthworms
contaminated with estrogen sang five times
as long as the starlings on a controlled diet.
They also sang more complex songs.
Starling experiment
Independent: Diet contaminated or not.
Dependent: Length and complexity of
songs.
Controlled: Starlings
Uncontrolled: Age/gender of bird.
How does brain size compare
in taxi versus bus drivers?
The taxi drivers drove different routes
every day.
The bus drivers drove the same route
every day.
Discovery: Taxi drivers has a larger
spatial memory (the part of your brain
that helps you navigate) than bus
drivers.
Variables
Independent variable: Taxi versus Bus
Driver
Dependent: Spatial memory
Controlled: Same city
Uncontrolled: Intelligence, Years of
driving experience.
Do marigolds planted in moon rocks
grow best with or without microbes?
Two groups of marigolds were planted
in crushed rock that is very similar to
rocks found on the moon.
Group 1: Microbes were added to the
soil.
Group 2: No microbes were added
Discovery:The marigolds with microbes
added to the soil grew best!
Independent: Microbes in soil or not
Dependent: Plant height
Controlled: same amount of soil and
water
Uncontrolled: health of the plant
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?