Designing Good Experiments Do Now: Please copy the following definitions onto your notes. (On New Week 2 Do Now sheet in notebooks, write “9.8 – copied definitions” PASS FORWARD DO NOW WEEK 1, please!) Hypothesis: A possible explanation for something. Prediction: An if… then… statement used to test a hypothesis. Dependent Variable: The variable in an experiment which is measured as a result. It’s value depends on the value of the independent variable. Independent Variable: The variable in an experiment whose values are selected by the experimenter. Finding the effect that changing this variable has is the purpose of an experiment. Control Group: The samples or trials in an experiment that DO NOT get any experimental treatment. It is what the other groups will be compared to.

Hypotheses answer questions How long can a plant survive without light? How many times can a planarian’s head be divided? How will a population of butterflies be affected if their habitat is turned into farmland?

Prediction: An if… then… statement that tests a hypothesis Hypothesis: Mice are able to learn how to solve a maze. Prediction: If [mice are able to learn how to solve a maze] then [mice will be able to find the cheese in a maze faster after having practice.] –If [hypothesis]… –Then [expected results if hypothesis is accurate].

Know your Variables! Jeff wants to figure out the effect of using hand sanitizer on the number of bacteria on his hands. He will measure the number of bacteria by putting his hand on a petri dish and then counting how many bacteria have grown there 3 days later. He does three trials of the experiment: one without using any sanitizer, one with “brand x” sanitizer, and one with “brand y.” What are the independent and dependent variables of the experiment? What is the control group?

Milgrim’s Weird (& Disturbing) Experiment As we watch the video about the famous Milgrim psychology experiment, ask yourself: –What is the independent variable? –What is the dependent variable? –What is the control group? / Is there a control group?

Designing Experiments A useful construct for experimental design is given by: –The effect of “INDEPENDENT VARIABLE” on “DEPENDENT VARIABLE”

Refresher Independent Variable: selected by experimenter trying to figure out what effect it has. Dependent Variable: measures the effect of the independent variable. Control Group: gets no experimental treatment. Independent variable = “0”

Consider: In an experiment to determine how different amounts of water will affect plant growth: –What is the IV? –What is the DV? –What is the control group? –What might a graph of the results look like?

Assignment Complete the Simpsons Science assignment in class today (now). If you do not finish, complete for homework.