1. Variables in Science Experiments
What makes an experiment “fair”?

2. Science experiments use…
Manipulated Variable: the one and only factor that is changed in an experiment.
Responding 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

3. A birdy example…
Imagine you want to see what color of bird feeders your local birds preferred.
Red? Blue? Green?
If a student wanted to determine whether feeder color has an effect, he might suggest, “if I put up a red, blue, and green feeder, birds will visit the green feeder more” as a hypothesis. He might speculate that the green feeder, being the most “camouflaged” or “natural” might be visited the most.

4. Variables Manipulated Variable: color of the feeders
Responding 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
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

5. Our Experimental Design
Controlled variables
Location of feeders
Kind of seed
Type of feeder
Manipulated
Variable
Red
Blue
Green
Responding
Amount of Seed Eaten

6. The experiment is FAIR. (ONLY the manipulated variable can change!)
If everything except the manipulated variable is held constant, we can say:
The experiment is FAIR.
(ONLY the manipulated variable can change!)

7. Is Sam’s experiment fair? YES? NO?
Read the following scenario to the students:
Sam wants to know if birds prefer one color of feeder to another. He makes three feeders out of 2-liter bottles and paints one red, one blue, and one green. He fills the feeders with the same amount of sunflower seed, and plans to keep the feeders out for one week before measuring how much seed is eaten out of each. On Tuesday, he puts each of the feeders in his back yard: the red feeder in a large dead tree, the blue feeder he sits on the doghouse, and the green one he puts in a small bushy tree.
Is Sam’s experiment fair?

8. Why is it important to change only 1 thing at a time?
Collect answers…

9. If you don’t… then you won’t know what is causing your results.
Give an example: a girl wanted to know whether her dog being out in the backyard scared birds away. She thought it would, because the dog likes to bark a lot and has been known to chase little animals.
For 5 days when the dog was out, she counted for an hour.
For 5 days the dog wasn’t out, she counted for an hour.
After she collected the data, she noticed that birds are actually out MORE when the dog is in the yard. Does that surprise you?
But, what the girl didn’t think of was other things that might effect the number of birds that were out. She began thinking that maybe something else might be going on. She realized that the dog was out on sunny warm days, and not on rainy, cool days.
So, even though she found a change, how will she know if it was the dog may have had an effect? Or was it the weather?
See if kids can suggest improvements to the experiment. (i.e. making sure the weather and temperature is similar on the days she counts.)
Independent Variable

10. 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.

11. Variables manipulated: Microwaved food or not
Responding : time it takes to get through the maze
Controlled: Fish species, maze, amount of food, type of food.

12. 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.

13. Smither’s experiment Manipulated variable: Juice or not
Responding 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.

14. How does diet affect the length and complexity of starling bird songs?

15. Contaminated versus Non-contminated 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.

16. Starling experiment Manipulated: Diet contaminated or not.
Responding: Length and complexity of songs.
Controlled: Starlings
Uncontrolled: Age/gender of bird.

17. 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.

18. Variables Manipulated variable: Taxi versus Bus Driver
Responding : Spatial memory
Controlled: Same city
Uncontrolled: Intelligence, Years of driving experience.

19. 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!

20. Manipulated: Microbes in soil or not
Responding : Plant height
Controlled: same amount of soil and water
Uncontrolled: health of the plant

21. 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?
You may want to have students brainstorm in pairs, or conduct a group discussion.
How would you advise her to find out if the expensive seed is better?

22. Terry’s Experimental Design
Manipulated
Variable ?
Responding
Constants
What is the Independent Variable?
What is the Dependent Variable?
What would you hold constant?

23. Terry’s Experimental Design
Independent
Variable
Kind of seed:
Expensive Cheap
Dependent
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 is the Independent Variable?
What is the Dependent Variable? Number of birds– also, you may want to record what KINDS visit. Maybe diversity is higher with one seed or the other!
What would you hold constant? There are other answers as well. Feel free to add to the list

24. What experiments are you planning?
What are your variables?
How will you keep your experiment fair?