Planning Fair Tests — Plan a fair test for a model or prototype. - 3rd Grade Science

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Why Do We Need Fair Tests?

People have been building and testing things for a very long time. Imagine you built a paper airplane and wanted to know if it flies farther than your friend's airplane. You would need to throw both planes the same way, from the same spot. If you threw yours really hard and your friend's gently, that would not be fair! A fair test is a test where you only change one thing at a time so you can trust your results.

1400s • Leonardo da Vinci Tests Designs

The famous inventor Leonardo da Vinci drew plans for flying machines and bridges. He tested small models before building big ones.

1800s • Engineers Test Bridges

Engineers began carefully testing bridge designs. They learned they had to keep tests fair to know which design was strongest.

1903 • The Wright Brothers Test Airplanes

Orville and Wilbur Wright tested over 200 wing shapes. They changed only one thing at a time to find the best wing design.

Today • Scientists and Engineers Use Fair Tests Every Day

From testing new sneakers to building rockets, fair tests help people figure out what works best.

All of these inventors had the same question: How do I know my design is actually better? The answer is to plan a fair test. If a test is not fair, you cannot trust the results. Let's learn how to plan one!

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Key Ideas Behind Fair Tests

Before you run a fair test, you need to understand a few important ideas. These ideas help you plan your test so that you can learn something real about your model or prototype (a first version of something you are building).

One Change at a Time

In a fair test, you change only one thing. This is called the variable you change. Everything else stays the same.

Keep Everything Else the Same

The things you keep the same are called controlled variables. They make the test fair because nothing extra changes.

Measure What Happens

You watch and measure what happens because of the change. This is called the result or outcome. You write it down so you can compare.

Test More Than Once

Do your test several times. If you get the same result each time, you can be more sure it is correct. Scientists call these repeated trials.

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KEY TAKEAWAY

Think of a fair test like a race. In a running race, all runners start at the same line, run the same distance, and start at the same time. If one runner got a head start, that would not be fair! In a fair test, everything is equal except the one thing you change.

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See How a Fair Test Works

Let's look at an example. Imagine you built two ramps and you want to test which ramp surface makes a toy car roll farther. The picture below shows how to set up a fair test.

Both ramps are the same height and use the same car. The only thing that is different is the ramp surface. You measure how far the car rolls on the floor after it leaves each ramp.

Look at the diagram above. Notice how both ramps are the same height. The same toy car sits at the top of each one. You let go of the car the same way each time — no pushing! The only difference is the ramp surface. That is the one thing you change. This makes the test fair.

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Steps to Plan a Fair Test

Planning a fair test is like following a recipe. If you follow the steps in order, your test will be set up the right way. Here are the steps you should follow every time.

Step-by-Step Guide

Step 1 — Ask a question. What do you want to find out about your model or prototype? For example: "Does a taller ramp make the car go farther?"
Step 2 — Pick the one thing you will change. This is your variable you change. In our example, it is the ramp height.
Step 3 — Decide what you will measure. This is the result. In our example, you measure how far the car rolls.
Step 4 — List everything you will keep the same. These are your controlled variables. Same car, same floor, same way of letting go.
Step 5 — Plan to test more than once. Do at least 3 trials so your results are trustworthy.
Step 6 — Record your data. Write down your measurements in a table so you can compare them.

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Remember!

If you change more than one thing at the same time, you will not know which change caused the result. That is why we always change only one thing!

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Understanding the Three Types of Variables

When scientists talk about fair tests, they use the word variable a lot. A variable is anything in your test that can change. There are three kinds, and you need to know all three to plan a fair test.

WHAT YOUMEASUREThe result youobserve or measureExample:how far the carrolls (in cm)📏WHAT STAYSTHE SAMEEverything elsestays equalExamples:same car, samefloor, no pushing🔒A fair test changes only ONE thing, measures ONE result, and keeps EVERYTHING ELSE the same.

The three boxes show the three types of variables. The variable you change (pink) is on the left. The result you measure (yellow) is in the middle. The controlled variables (purple) are on the right.

Look at the three boxes in the diagram. Every fair test has all three types of variables. If you can fill in all three boxes before you start your test, you are ready to go! Try this: whenever you plan a test, draw three boxes on your paper and label them "Change," "Measure," and "Same." Fill them in, and your test will be fair.

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Worked Example: Testing a Parachute

Let's walk through a real example. Your class built parachutes out of plastic bags. You want to find out if a bigger parachute falls more slowly than a smaller one.

Planning a Fair Test for a Parachute

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Step 1 — Ask a Question

Does a bigger parachute fall more slowly than a smaller parachute?
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Step 2 — Identify What You Will Change

The size of the parachute is the variable you change. You will test a small parachute (20 cm × 20 cm) and a large parachute (40 cm × 40 cm).
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Step 3 — Identify What You Will Measure

You will measure the time it takes to fall from the top of a staircase to the floor. You will use a stopwatch.

Result to measure: fall time in seconds
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Step 4 — List Controlled Variables

You keep these the same: same material (plastic bag), same string length, same weight attached, same drop height, same person dropping it.

5 things stay the same
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Step 5 — Plan Repeated Trials

You will drop each parachute 3 times. Then you find the middle number to get a good answer.

3 trials for each size
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Step 6 — Record Data in a Table

Make a table with columns for the parachute size, trial number, and fall time. After testing, you can compare the times.

Parachute Size	Trial 1	Trial 2	Trial 3
Small (20 cm × 20 cm)	2.1 seconds	1.9 seconds	2.0 seconds
Large (40 cm × 40 cm)	3.5 seconds	3.3 seconds	3.4 seconds

Sample data from the parachute fair test

The large parachute took about 3.4 seconds to fall, while the small parachute took about 2.0 seconds. Because the test was fair, you can be confident that the bigger parachute really does fall more slowly!

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Fair Tests vs. Unfair Tests

How can you tell if a test is fair or unfair? Let's compare some examples side by side.

Feature	Fair Test ✅	Unfair Test ❌
Things changed	Only one thing is different	Two or more things are different
Controlled variables	Everything else stays the same	Some things are different and you didn't notice
Number of trials	Test is repeated 3 or more times	Only tested once
Results	You can trust the results	You cannot be sure what caused the result
Example	Test two paper airplanes with same paper, same throw, but different wing shape	Test two paper airplanes with different paper, different throw, and different wing shape

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KEY TAKEAWAY

Think about baking cookies. If you want to know whether chocolate chips or raisins taste better, you should use the same recipe and the same oven temperature for both batches. The only thing you change is the mix-in (chips vs. raisins). If you also changed the oven temperature, you would not know if the taste was because of the mix-in or the oven!

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From Fair Tests to Real Engineering

Planning fair tests is a big part of the engineering design process. Engineers design something, test it, learn from the results, and then improve it. When you get older, you will plan even bigger and more complex tests. But the basic idea stays the same: change one thing, keep everything else the same, and measure the result.

What You Learn Now	What Comes Next
Change one thing at a time	Design experiments with many variables
Do 3 trials	Use math to figure out averages and patterns
Write results in a simple table	Make graphs and charts to show data
Test a model or prototype	Test real products for safety and quality

Every time you plan a fair test now, you are practicing a skill that real engineers and scientists use every single day. Keep it up!

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Practice Problems

Question 1: CONCEPTUAL

What does it mean for a test to be "fair"? In your own words, explain why changing only one thing at a time is important.

Show answer

A fair test is one where you change only one thing and keep everything else the same. This is important because if you change two things at the same time, you won't know which change caused the result. For example, if you change both the size and material of a parachute, and it falls slower, you can't tell if it was the size or the material that made the difference. Changing only one thing lets you know for sure what caused the result.

Question 2: BASIC

Maya wants to test if a longer paper airplane flies farther than a shorter one. She plans to use the same paper, throw both planes the same way, and test in the same room. What is the variable she changes? What does she measure? Name two controlled variables.

Show answer

The variable Maya changes is the length of the paper airplane (long vs. short). What she measures is how far the airplane flies. Two controlled variables are: she uses the same paper for both planes, and she throws both planes the same way. She also tests in the same room, which is another controlled variable. Maya's plan is a fair test because only one thing is different between the two planes.

Question 3: INTERMEDIATE

Carlos built two toy boats. He wants to test which shape moves through water faster. He tests Boat A in a bathtub and Boat B in a swimming pool. He pushes Boat A gently and Boat B hard. Is this a fair test? Explain what Carlos should fix.

Show answer

No, this is not a fair test. Carlos changed three things: the boat shape, the place he tested (bathtub vs. pool), and how hard he pushed. To make the test fair, Carlos should test both boats in the same body of water, like the same bathtub. He should push both boats the same way, or use a ramp to launch them. The only thing that should be different is the boat shape. Then he can trust that any difference in speed is because of the shape, not the water or the push.

Question 4: APPLIED

Your class built two bridges out of craft sticks. Bridge A uses 20 sticks and Bridge B uses 40 sticks. You want to find out which bridge holds more weight. Write a complete plan for a fair test. Include: your question, the variable you change, what you measure, at least three controlled variables, and how many trials you will do.

Show answer

Question: Does a bridge made with 40 craft sticks hold more weight than a bridge made with 20 craft sticks? Variable you change: the number of craft sticks (20 vs. 40). What you measure: the amount of weight the bridge can hold before it breaks or bends. Controlled variables: (1) same type of craft sticks, (2) same type of glue, (3) same length of bridge, (4) same supports on each end, (5) weight placed in the same spot in the middle. Trials: Test each bridge 3 times. This plan is fair because the only thing that is different is the number of sticks.

Question 5: CRITICAL THINKING

Emma tested her windmill prototype three times. Her results were: Trial 1 = 5 spins, Trial 2 = 12 spins, Trial 3 = 6 spins. Trial 2 is very different from the other two. What might have gone wrong? What should Emma do next? How does this connect to the idea of a fair test?

Show answer

The result from Trial 2 (12 spins) is much higher than Trial 1 (5 spins) and Trial 3 (6 spins). Something probably went wrong during Trial 2. Maybe Emma blew harder on the windmill that time, or maybe a gust of air came from an open window. This means a controlled variable might not have stayed the same during that trial. Emma should do more trials (maybe 5 total) to see if 12 was just a mistake. She should also double-check that she is keeping everything the same each time, like how she blows on the windmill. This connects to fair testing because if a controlled variable changed without Emma noticing, the test was not truly fair for that trial.

SUMMARY

Lesson Summary

Summary

A fair test is an experiment where you change only one thing (the variable you change) and keep everything else the same (the controlled variables). You measure the result to see what happens. This helps you know for sure what caused the outcome. Engineers and scientists have used fair tests for hundreds of years, from the Wright Brothers testing airplane wings to engineers testing bridge designs.

To plan a fair test: (1) ask a question, (2) pick the one thing to change, (3) decide what to measure, (4) list what stays the same, (5) plan repeated trials, and (6) record your data in a table. Remember: if you change more than one thing, you will not know which change caused the result. Keep your tests fair, and your results will be trustworthy!