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.
    • 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.
    • 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?!