Experimental Design

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!!!

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.

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”

Dependent Variable (DV)
To determine the dependent variable, ask yourself:
“ What will I measure and observe?”
“ 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!

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

Controlled Variable (CV) To determine the controlled variables, ask yourself:
“ What should not be allowed to change?”
“ 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?!

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Experimental Design

by Jeremy Mularella on Sep 03, 2007

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Experimental Design — Presentation Transcript