Is That Test a Fair Test?

By the end of this lesson, you will be able to look at an experiment or a design and decide if it was done well.

Imagine your class wants to know which brand of paper towel soaks up the most spilled water. One group tests Brand A with one big cup of water. Another group tests Brand B with a tiny splash. Can you trust the answer? No. The test was not fair, because more than one thing changed. A variable is anything in a test that can change, like the amount of water, the size of the towel, or how long you wait. In a fair test, you change only one variable and keep everything else the same.

When you evaluate an experiment, you ask good questions about it. Did the scientists change only one thing? Did they measure carefully with a tool, like a graduated cylinder, instead of just guessing? Did they repeat the test more than once? One try can be luck. Three tries show a pattern you can trust.

Engineers need checking too. Suppose your team builds a shade shelter for a bench outside your Texas school in August. You would evaluate the design by asking if it meets the criteria, which are the goals the design must reach, like blocking sunlight and holding up in strong wind. If the shelter blows over the first windy afternoon, the design failed one goal. That is not a disaster. It is information. Engineers use what went wrong to make the next version better.

So evaluating means judging with evidence, not feelings. You do not say "I like that one." You say "This test changed two things at once, so the results are not reliable," or "This shelter met two goals but missed one."

Try it: A student tests two toy cars on a ramp, but rolls one from the top and one from the middle. What should she change to make it fair?