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Designing Good Experiments

Designing Good Experiments. Do Now: Please copy the following definitions into your notes. (On Do Now sheet in notebooks, write “9.2 – copied definitions”) Hypothesis : A proposed explanation for a phenomenon. Prediction: An if… then… statement used to test a hypothesis.

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Designing Good Experiments

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  1. Designing Good Experiments Do Now: Please copy the following definitions into your notes. (On Do Now sheet in notebooks, write “9.2 – 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. 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.

  2. 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 a Higgs boson, 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)

  3. 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?

  4. 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].

  5. 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.”

  6. 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?

  7. 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

  8. 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

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

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

  11. 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.

  12. 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

  13. 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?

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

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