Variables: Independent, Dependent & Controlled

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Variables: Independent, Dependent & Controlled

Postby kjud » Thu Jan 17, 2008 7:53 pm

Hi there,

I am helping my daughter w/ her science fair project & as we were reviewing the information on variables, the expanation mentions that good projects have only one independent, one dependent & one controlled variable. What my daughter has planned to test are the best conditions for handwashing. We wanted to do experiments that test the duration of handwashing (15 sec. vs. 30 sec), the temperature of the water (cold, warm) and the use of soap (with or with out soap).

She has the independent variable as the water temperature (that will be measured and controlled with a thermometer to stay consistent). She has the dependent variable as the amount of "germs" that will be put on her hands for each experiment; will be measured to ensure consistency.
**As we can't "measure" the exact amount of the "germs" left on her hands after each test, we planned to take pics and compare each result & then rank which set of conditions left the most germs on the hands, thus the test that showed the least amount of germs left on the hands would be ranked as the best conditions to get rid of germs, and recommend that to get the most germs off your hands while wahsing, to repeat those conditions. She has the controlled variable as the amount of soap, which will be 1 full pump each test.

Are these correct? [color=#0000FF]Where do we fit in the time variance of 15 seconds vs. 30 seconds?

Thanks for your guidance:)

Kristi & Michaela Jud[/color]
Posts: 2
Joined: Thu Jan 17, 2008 7:28 pm
Occupation: parent
Project Question: What are the ideal hand washing conditions to get rid of germs
Project Due Date: 1/18/08
Project Status: I am just starting

Re: Variables: Independent, Dependent & Controlled

Postby Grace » Thu Jan 17, 2008 10:27 pm

Hi Kristi and Michaela,

This sounds like a great project to pursue. However, I am unsure whether photographs would be able to capture the relative changes in microbes produced by the different methods of hand-washing. An alternative way of collecting data would be to use agar plates.

You can read more about how agar plates may be used on this page: ... ?from=Home
Though the above link is addressing a different experiment, the agar plate and colony-counting methods for assessing bacterial content can be used for your project, too!

You are correct about the *dependent variable* being the amount of germs left on the hands after washing.

Your proposed experiment has multiple *independent variables*: (1) duration of hand-washing; (2) water temperature; (3) use of soap.

Since you have three independent variables, your experiment will require eight trials:
15 sec, cold, with soap
30 sec, cold, with soap
15 sec, cold, without soap
30 sec, cold, without soap
15 sec, warm, with soap
30 sec, warm, with soap
15 sec, warm, without soap
30 sec, warm, without soap

This will require careful use of controls. You will want to make sure you keep the following the same (controlled) for all eight trials: (1) person who is doing the hand-washing; (2) method of "dirtying up" the hands before washing; (3) method of hand-washing (washing between the fingers, the backs of the hands, or just the palms, etc.); (4) new, clean towel for drying hands.

Hope this helps! Let us know if you have further questions.

Grace Lin
Former Expert
Posts: 19
Joined: Thu Oct 18, 2007 8:37 pm

Re: Variables: Independent, Dependent & Controlled

Postby kjud » Fri Jan 18, 2008 2:46 pm

Thank you so much for your detailed response! VERY helpful.

We will look into the agar plates as a way to assess the bacterial count. For "germs" we are using a product called "glo germ", that, if properly used, shows up using a "black light", thus producing an image of the amount of germs left on the hands. Perhaps we will just use the pics as a visual aid?

Thanks again for all your help!

We will keep you posted as to how this goes!!

Have a great 3 day weekend:)

Kristi & Michaela
Posts: 2
Joined: Thu Jan 17, 2008 7:28 pm
Occupation: parent
Project Question: What are the ideal hand washing conditions to get rid of germs
Project Due Date: 1/18/08
Project Status: I am just starting

Re: Variables: Independent, Dependent & Controlled

Postby jeffreyshu » Thu Jan 24, 2008 11:19 pm

For a science fair project, you might be able to get away with just showing pictures especially if the results are dramatically different from one trial to another. A proper scientific paper would require you to express your results as numbers and to do statistical calculations to back up your conclusions. Doing bacterial experiments with agar plates is traditional way health officials use to do bateria counts. In your case, you could quantify your bateria as a percentage instead of a bateria count.

I think your "glo germs" are a much more fun way to see the effects of hand washing. Since it's visual, you'll be able to better communicate your results to the judges and to yourself. You will have a better time with the experiment from seeing and showing your bacteria instead of showing just the numbers.

One note of caution. Before you get your hands soaked into bacteria, make sure you have a camera or setup that can take pictures in the dark. I've seen some exibits that didn't come out as planned because their camera/film wasn't sensitive enough or their camera setup wasn't stable enough to capture low level flourescence. Good luck.
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Joined: Mon Sep 24, 2007 9:36 am

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