Variables in Science Experiments What makes an experiment “fair”?
Variables l l A variable is something that is changed A constant, or control, is something that does not change
Science experiments use… l l l 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 l l l 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
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 l Location of feeders l. Type of feeder l. 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?