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# Experimental Design

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How to conduct a valid experiment in a middle school science class.

How to conduct a valid experiment in a middle school science class.

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### Transcript

• 1. Experimental Design How to conduct a valid experiment.
• 2. A Good Experiment
• Tests one variable at a time. If more than one thing is tested at a time, it won’t be clear which variable caused the end result.
• Must be fair and unbiased . This means that the experimenter must not allow his or her opinions to influence the experiment.
• Does not allow any outside factors to affect the outcome of the experiment.
• 3. A Good Experiment
• Is valid . The experimental procedure must test your hypothesis to see if it is correct.
• If the procedure does not test your hypothesis, the experiment is not valid and the data will make no sense!
• Has repeated trials . Repeating the trials in the experiment will reduce the effect of experimental errors and give a more accurate conclusion.
• 4. 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 the experiment.
• There are three main types of variables.
• 5. 3 Kinds of Variables
• Independent Variable (IV)
• – something that is intentionally changed by the scientist
• What is tested
• What is manipulated
• Also called a “Manipulated Variable”
• You can only change ONE variable in an experiment!!!
• 6. 3 Kinds of Variables
• Independent Variable (IV)
• To determine the independent variable, ask yourself:
• “ What is being changed?”
• Finish this sentence…
• “ I will change the _____________”
• 7. Independent Variable
• Levels of the IV
• These are different ways you will change the independent variable
• Example: Assume you are testing five brands of popcorn to see which has the most unpopped kernels.
• The IV would be the different brands of popcorn.
• The five different brands would be the different levels of the IV.
• 8. 3 Kinds of Variables
• Dependent Variable (DV)
• – something that might be affected by the change in the independent variable
• What is observed and measured
• The data collected during the investigation
• Also called a “Responding Variable”
• 9. 3 Kinds of Variables
• Dependent Variable (DV)
• To determine the dependent variable, ask yourself:
• “ What will I measure and observe?”
• Finish this sentence…
• “ I will measure and observe ________________”
• 10. Dependent Variable
• Operational Definition:
• Define exactly how the dependent variable will be measured.
• Example: Assume your DV in an experiment is “plant growth.” How will you measure this?! It could be…
• Height (cm), mass (g), # of leaves, etc.
• Be specific and include all necessary units!
• 11. 3 Kinds of Variables
• Controlled Variable (CV) – a variable that is not changed and kept the same
• Also called constants
• Allows for a “fair test”
• NOT the same as a “control”!!
• Any given experiment will have many controlled variables
• 12. 3 Kinds of Variables
• Controlled Variable (CV) To determine the controlled variables, ask yourself:
• “ What should not be allowed to change?”
• Finish this sentence…
• “ I will not allow the ______________ to change.”
• 13. Control
• A group or individual in the experiment that is not tested, but is used for comparison as a reference for what “normal” would be like.
• Not all experiments have a control (though all experiments have controlled variables).
• Example: If you tested different pollutants to see their affect on plant growth, the control would only receive water.
• 14. Here are some different examples:
• 15. Students of different ages were given the same jigsaw puzzle to put together. They were timed to see how long it took to finish the puzzle.
• 16. Identify the variables in this investigation!
• 17. What was the independent variable?
• Ages of the students
• Different ages were tested by the scientist
• 18. What was the dependent variable?
• The time it to put the puzzle together
• The time was observed and measured by the scientist
• 19. What was a controlled variable?
• Same puzzle
• All of the participants were tested with the same puzzle.
• It would not have been a fair test if some had an easy 30 piece puzzle and some had a harder 500 piece puzzle.
• 20. Another example:
• 21. An investigation was done with an electromagnetic system made from a battery and wire wrapped around a nail. Different sizes of nails were used. The number of paper clips the electromagnet could pick up was measured.
• 22. What are the variables in this investigation?
• 23. Independent variable:
• Sizes of nails
• These were changed by the scientist.
• They used different sizes of nails in their experiment to see what effect that would have.
• 24. Dependent variable:
• Number of paper clips picked up
• The number of paper clips were observed and counted (measured)
• 25. Controlled variables:
• Battery, wire, type of nail
• None of these items were changed
• They had used the same battery, same wire, and same type of nail.
• Changing any of these things would have made it an unfair test.
• 26. Here’s another:
• 27. The temperature of water was measured at different depths of a pond.
• 28.
• Independent variable – depth of the water
• Dependent variable – temperature
• Controlled variables – same pond; same thermometer
• 29. Last one:
• 30. Students modified paper airplanes by cutting pieces off, adding tape, or adding paper clips to increase the distance thrown.
• 31.
• Independent variable – weight of plane, center of gravity, air resistance (depended on student choice-but only one was tested)
• Dependent variable – distance thrown
• Controlled variables – same plane design; same paper; same throwing technique
• 32. Now let’s take what we know about these variables and use them in an experiment!
• 33. We are going to test how many drops of water will fit on different sized coins.
• Let’s think about how we could test this.
• Identify the variables
• What exactly will be changed? How will it be changed?
• What exactly will be measured? How will it be measured?
• 34.
• Independent variable – size of the coin (penny, nickel, dime, quarter)
• Dependent variable – amount of water held on coin (# of drops)
• Controlled variables
• Same eye dropper
• Same water
• Same side of coin (pick heads or tails)
• Same technique (height/angle of dropper)
What are my variables ?
• 35. Are there any questions?!