Experimental Design

Slide 1: Experimental Design How to conduct a valid experiment.

Slide 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.

Slide 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.

Slide 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. 

Slide 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!!!

Slide 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 _____________”

Slide 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.

Slide 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” –

Slide 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 ________________”

Slide 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! 

Slide 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

Slide 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.”

Slide 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.

Slide 14: Here are some different examples:

Slide 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.

Slide 16: Identify the variables in this investigation!

Slide 17: What was the independent variable? Ages of the students Different ages were tested by the scientist –

Slide 18: What was the dependent variable? The time it to put the puzzle together The time was observed and measured by – the scientist

Slide 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.

Slide 20: Another example:

Slide 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.

Slide 22: What are the variables in this investigation?

Slide 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.

Slide 24: Dependent variable: Number of paper clips picked up The number of paper clips were observed – and counted (measured)

Slide 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.

Slide 26: Here’s another:

Slide 27: The temperature of water was measured at different depths of a pond.

Slide 28: Independent variable – depth of the  water Dependent variable – temperature  Controlled variables – same pond;  same thermometer

Slide 29: Last one:

Slide 30: Students modified paper airplanes by cutting pieces off, adding tape, or adding paper clips to increase the distance thrown.

Slide 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

Slide 32: Now let’s take what we know about these variables and use them in an experiment!

Slide 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?

Slide 34: What are my variables? 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) –

Slide 35: Are there any questions?!