Experimental Design
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Experimental Design



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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Experimental Design Experimental Design Presentation Transcript

  • Experimental Design How to conduct a valid experiment.
  • 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.
  • 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.
    View slide
  • 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.
    View slide
  • 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!!!
  • 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 _____________”
  • 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.
  • 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”
  • 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 ________________”
  • 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!
  • 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
  • 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.”
  • 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.
  • Here are some different examples:
  • 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.
  • Identify the variables in this investigation!
  • What was the independent variable?
    • Ages of the students
      • Different ages were tested by the scientist
  • What was the dependent variable?
    • The time it to put the puzzle together
      • The time was observed and measured by the scientist
  • 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.
  • Another example:
  • 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.
  • What are the variables in this investigation?
  • 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.
  • Dependent variable:
    • Number of paper clips picked up
      • The number of paper clips were observed and counted (measured)
  • 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.
  • Here’s another:
  • The temperature of water was measured at different depths of a pond.
    • Independent variable – depth of the water
    • Dependent variable – temperature
    • Controlled variables – same pond; same thermometer
  • Last one:
  • Students modified paper airplanes by cutting pieces off, adding tape, or adding paper clips to increase the distance thrown.
    • 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
  • Now let’s take what we know about these variables and use them in an experiment!
  • 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?
    • 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 ?
  • Are there any questions?!