Engineering MakingTesting Your Design with a Fair Test
Atlas the friendly explorer stands at a workshop table, holding two paper parachutes — each with a small paper canopy attached to a binder clip — while a stopwatch and an open notebook of recorded results sit in front of him.
- Define a fair test as changing only one thing at a time while keeping everything else the same.
- Identify which variables must stay the same to keep a parachute test fair.
- Record results in a notebook after each test drop.
- Compare test results to the design criteria to judge how well a prototype works.
- Explain how a failed result reveals failure points that guide the next improvement.
Key terms
- prototype
- a first model you test
- fair test
- a test that changes only one thing
- results
- what you write down after a test
- failure point
- the part of a design that fails
Test Just One Thing
Engineers build a first model called a prototype, then they test it fairly. A fair test changes only one thing at a time and keeps everything else the same. To see if a bigger canopy falls slower, you drop a small-canopy and a big-canopy parachute from the same height, with the same clip weight, in the same room. Only the canopy size changes. If you changed two things at once, you would not know which one mattered.
Write It Down and Check
After each drop, write the result in a notebook, like how many seconds it took to land. Writing results down gives you real evidence, not just a memory or a guess. Then you compare your results to your design criteria. If your criterion was three seconds and the parachute took only two, that is useful. It shows the failure point, the part that needs work. Finding failure points is good, because it tells you what to fix next.
Worked examples
You want to know if a bigger canopy falls slower. What stays the same?
- Keep the drop height the same.
- Keep the clip weight the same.
- Only change the canopy size.
Answer: Drop height and clip weight stay the same.
Your parachute fell in 2 seconds but needed 3. What now?
- See the result as a useful clue.
- Change one thing, like a bigger canopy.
- Test it again.
Answer: Find the failure point, change one thing, and test again.
Activity
Look at each parachute test below and decide: which tests are fair tests of 'does a bigger canopy fall slower?'
Practice
List what must stay the same in a fair canopy test.
Write down a test result and compare it to the criterion.
Common mistakes to avoid
- Change many things at onceChanging many things hides which one caused the result.
- A failed test means give upA failed test is useful and shows what to improve.
Check your understanding
What makes a test of a prototype a 'fair test'?
Atlas wants to know if a bigger canopy falls slower. What should stay the SAME for both parachutes?
Why does Atlas write the seconds in a notebook after each drop?
Atlas's criterion was 'fall for at least 3 seconds.' His parachute took only 2 seconds. What is the best next step?
Recap
Engineers test a prototype with a fair test that changes only one thing. They write results down for real evidence and compare them to the criteria. A result below the goal shows a failure point, which tells them what to fix next.
Reflect
What would you write down during your own fair test?