🔁 Why every test needs three tries

What one try can't tell you

Say you drop the ball once on each floor, from 100 cm, and write down how high it bounces. Here are some made-up numbers:

• Carpet: 57 cm.
• Hard floor: 55 cm.
• Folded towel: 20 cm.

Carpet beat the hard floor! Or did it? Maybe the ball landed on a lump in the carpet. Maybe you let go a little higher that time, or read the tape a moment late. With one try, you can't tell a real result from a fluke, which is an accident that happens once.

What three tries show

Now drop it three times on each floor. These numbers are made up too:

• Carpet: 57, 36 and 39 cm.
• Hard floor: 55, 52 and 58 cm.
• Folded towel: 20, 23 and 17 cm.

Look at the carpet. Two tries are close together, and one is way off. That odd one is the fluke. On the other floors the tries sit close to each other, and that closeness is what makes you trust them.

Tries that land close together are a good sign that you did each one the same way: the same height, the same letting go, the same way of reading the tape. Tries that jump about tell you to slow down and check how you're doing it.

Three tries can't make a test perfect. What they do is let you see when one number doesn't belong.

Finding the middle number

From 4th grade up, you find the mean for each floor: add the three tries, then divide by 3.

• Carpet: 57 + 36 + 39 = 132, and 132 ÷ 3 = 44 cm.
• Hard floor: 55 + 52 + 58 = 165, and 165 ÷ 3 = 55 cm.
• Folded towel: 20 + 23 + 17 = 60, and 60 ÷ 3 = 20 cm.

Even with the fluke inside it, the carpet's mean comes out below the hard floor's. One wild try moves the mean a little, but it can't flip the answer the way it did when you had only one try.

If one try is clearly a mistake, like the ball clipping the wall, don't just cross it out. Draw a line through it, do that try again, and write in your logbook what happened. Judges like hearing about that.

How many is enough?

Every one of the 118 projects here asks for at least three tries at every level. Three is the usual minimum at school fairs, and more is better if you have time: five tries make the mean steadier still. Here's what fairs usually expect at each age:

• 1st–3rd: Three at each level (one is fine at a first fair).
• 4th–6th: Three at each level.
• 7th and up: Three or more at each level.

Write every try in the results table the moment you measure it, and read how to make a results table to see where the mean goes.