Designing Good Experiments Do Now: Please copy the following definitions onto the notes outline. (On Do Now sheet in notebooks, write “8.31 – copied definitions”) Hypothesis: A proposed explanation for a phenomenon. Prediction: An if… then… statement used to test a hypothesis. Dependent Variable: The variable in an experiment which is measured as a result. Its 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.

Characteristics of a Scientific Hypothesis Scientific hypotheses must have three key characteristics TESTABLE: Some measurement must exist that could support or refute a given hypothesis. (e.g. particle physicists hypothesize the existence of vibrating subatomic strings, but no technology yet exists that has found it.) FALSIFIABLE: There must be some possible measurement that would refute the hypothesis. (e.g. finding just one fossil that is not in the correct time period – such as Precambrian rabbits – would refute many evolutionary hypotheses) MECHANISM: A good hypothesis includes some explanation as to how something happens (e.g. falling objects move toward the Earth because of gravitational attraction)

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: Logical 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].

Uncertainty: Do we ever know anything 100%? A hypothesis can never be “proved” to 100% satisfaction. Only in theoretical mathematics can one achieve “proof.” Although data may support a hypothesis, there is always the possibility that some yet-undiscovered phenomenon will refute it. In the real world, 100% proof is an impossibility, but being 95-99+% sure is described as “certain.”

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? http://www.youtube.com/watch?v=y6GxIuljT3w

Designing Experiments A useful construct for experimental design is given by: The effect of “X” on “Y” or The response of “Y” to changes in “X” Where X = IV, and Y = DV

Variables & Graphing The X (horizontal) axis is ALWAYS the independent variable (IV) The Y (vertical) axis is ALWAYS the dependent variable (DV)

The effect of [IV] on [DV] Example: Enzyme Kinetics The effect of substrate concentration on reaction velocity IV = ? DV = ? The effect of [IV] on [DV] f(x) = y DV = f(IV)

Constants = controlled variables A variable that is controlled remains constant over the course of the experiment. The goal of experimental design is to control as many variables as possible. Ideally, only the IV and DV are not controlled.

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” Graphs: The effect of IV on DV. DV = f(IV) Constants = controlled variables

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.