How to design a valid experiment Experimental Design How to design a valid experiment

What makes a good experiment? Test only one variable at a time. Must be fair and unbiased. Does not allow outside factors to affect what happens. Is Valid: tests your hypothesis.

What are variables? A variable is anything in an experiment that can change or vary. It is any factor that can have an effect on the outcome of an experiment. There are three main types of variables.

3 kinds of variables Independent Variables Something that is intentionally changed. Reminder: You only want to change ONE variable in an experiment.

3 kinds of variables Independent Variables ”I will change _______________________.”

Independent Variables z Independent Variables Example: You are testing 5 brands of popcorn to see which has the most unpopped kernels after 2 minutes. The independent variable is the different kinds of popcorn.

3 kinds of variables Dependent Variables Something that might be affected by the change in the independent variable. This is the thing that you observe or measure.

3 kinds of variables Dependent Variables ”I will measure _______________________.”

z Dependent Variables What is the dependent variable in the popcorn example?

z Dependent Variables If I want to find out an effect of fertilizer on plant growth, what is the dependent variable? How should I measure it?

3 kinds of variables Controlled Variables A variable that is not changed that is kept the same. Not the same as the control. Makes the test fair. Experiments will probably have many controlled variables.

3 kinds of variables Controlled Variables ”I will not allow the __________ to change.”

Control A group or individual in the experiment that is not tested, but is used to see what “normal” would be like. Not all experiments have a control, but all have controlled variables.