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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 Earth’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 “water” or “land”.  Make sure the record keeper can keep up with you! ☺
  • Toss the globe for about 50 tosses.
  • Create a 95% confidence interval for the true proportion of water on the Earth’s surface.
  • If we would like to be able to estimate the true proportion of water on the Earth’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 Earth’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 EPA’s approximation?  Test an appropriate hypothesis.
Again... simple, fun, and educational!

~SSB


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