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Test for Proportions: Water on Earth

This is such a simple, yet fun activity!  The only thing you need is a blow-up globe beach ball (cheap on Amazon).  I have no idea where I first saw this, but I'm so glad I found it!

When doing this activity, the students have already been introduced to confidence intervals for proportions.  And I use this as my lead-in to significance tests for proportions.  Here is what I have projected for the class (use animations so it's not all on the screen at once).
  • Our first goal is to determine a guess for the amount of water on the Earthā€™s surface.
  • Stand (or sit) in a circle and toss the globe back and forth.  After each catch, check the tip of your index finger from your dominant hand and call out ā€œwaterā€ or ā€œlandā€.  Make sure the record keeper can keep up with you! ā˜ŗ
  • Toss the globe for about 50 tosses.
  • Create a 95% confidence interval for the true proportion of water on the Earthā€™s surface.
  • If we would like to be able to estimate the true proportion of water on the Earthā€™s surface to within 3%, with 95% confidence, how many times should we toss the globe?
  • Our second goal is to check on the accuracy of our globe.  According to the Environmental Protection Agency, the amount of water on the Earthā€™s surface is close to 71%.  Obviously, I did not invest in the highest quality globes available. At an alpha of 0.05, is the globe consistent with the EPAā€™s approximation?  Test an appropriate hypothesis.
Again... simple, fun, and educational!

~SSB


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