Experimental Design [1.3]• SPI 5 Defend a conclusion based on scientific

evidence.• SPI 7 Compare conclusions that offer different,

but acceptable explanations for the same set of experimental data.

Goals:• Learn the components of a formal experiment• Design a formal experiment to test something

of your choice

Experiments• A controlled experiment tests the predictions

of a hypothesis.• By controlled, we mean the experiment has 2

groups held under the same conditions.– Control Group– Experimental Group

…more on these 2 groups in a minute.

Experimental Design…

• Dependent Variable: the factor or outcome we are measuring; what happens with it depends on what we do with the independent variable.

• Independent Variable: The factor that we are directly controlling; changing it directly affects the dependent variable/outcome of the experiment.

…Experimental Design

• Experimental group: This is the group that we mess with. The independent variable is manipulated in this group to see if a change occurs in the dependent variable. • Sometimes called the treatment group because they

“receive the treatment”.

• Control group: We don’t mess with this group. It is used for comparison; the independent variable is either absent, held constant, or replaced with a placebo.

An Example Experiment:You want to see whether a plant grows taller with a certain type of fertilizer.• Take two plants, add the fertilizer to one and don’t

add it to the other.• Make sure that each plant gets the same amount of

water, sunlight, and heat• Measure the growth of each plant over time.

Experimental Group Independent Variable

Plant that gets the fertilizer Fertilizer

Control Group

Plant that doesn’t get fertilizer

Dependent Variable

Height of plant

An Example Experiment:Your dog’s breath STINKS. You think switching his food might help him not have breath that would kill a small elephant.• Switch his dog food to a different brand.• Make sure everything else he does stays as close to

the same routine as possible.• Let a week pass and take a whiff.

Experimental Group Independent Variable

Your dog The dog food

Control Group

Technically not one, but you’re controlling for as many other factors as possible

Dependent Variable

Dog breath

Design You Own Experiment

Pick a product, a claim you have heard, or something else you might like to ask a question about and include the following:• Write a hypothesis that makes a prediction

(example: Product X does ______ better than product Y)

• Detail how you are going to test your hypothesis. Include your experimental group, control, independent and dependent variable.