Plan a Fair Scientific Test

A scientific question becomes much more useful when you can turn it into a test that produces evidence.

In this lesson you will plan a fair scientific test from beginning to end.

You will choose a measurable question, identify the variables, decide what to keep consistent, plan measurements, write a repeatable procedure and decide how the results will be recorded.

Goal: create a clear investigation plan that another person could understand and repeat.

Before you start

Review The Scientific Method Explained Step by Step.

Also review Independent, Dependent and Controlled Variables.

You will need a notebook, document or worksheet for your investigation plan.

What makes a scientific test fair?

In a simple fair test, you deliberately change the factor you want to investigate while keeping other important conditions as consistent as practical.

You then measure what happens.

The purpose is to make the comparison easier to interpret.

A fair test does not mean that every detail in the environment must be identical.

Instead, you identify factors that could reasonably influence the result and control them where possible.

The basic planning pattern

A useful investigation plan answers these questions:

1. What do I want to find out?
2. What will I change?
3. What will I measure?
4. What important conditions will I keep the same?
5. What materials and equipment will I need?
6. What exactly will I do?
7. How many measurements or trials will I collect?
8. How will I record the results?
9. What safety issues must I consider?

Step 1: Start with a testable question

A useful experimental question should allow you to make a comparison and collect evidence.

Consider this question:

Do plants like light?

It is difficult to test because the words like and light are not defined precisely enough.

A clearer question could be:

How does the number of hours of light each day affect the height of seedlings after fourteen days?

Now the planned change and the measured outcome are much clearer.

Make the question measurable

Compare:

Does warm water dissolve sugar better?

with:

How does water temperature affect the time required for five grams of sugar to dissolve?

The second question tells us what can be measured.

Practice 1: Improve a question

Rewrite this:

Do bigger paper aeroplanes fly better?

You first need to decide what bigger and better mean.

One possible version is:

How does wing length affect the distance travelled by a paper aeroplane?

Other versions could be valid if the measurements are clearly defined.

Step 2: Identify the independent variable

Ask:

What factor will I deliberately change?

For the paper aeroplane investigation, the independent variable is wing length.

You should decide the values you will test before beginning.

For example:

• 8 centimetres
• 10 centimetres
• 12 centimetres
• 14 centimetres

The values should be practical, safe and useful for comparison.

Do not change several important factors at once

Suppose each paper aeroplane has a different wing length, different paper type and different overall design.

If one travels farther, it becomes difficult to identify which difference mattered.

For a simple fair test, change one main factor at a time.

Step 3: Identify the dependent variable

Ask:

What response will I measure or observe?

For the paper aeroplane example, the dependent variable is distance travelled.

You also need to decide how distance will be measured.

For example:

Measure from the launch line to the point where the aeroplane first touches the floor.

A clear measurement rule helps make different trials comparable.

A variable is not the same as its measuring tool

If distance is measured with a tape measure, the dependent variable is distance.

The tape measure is the instrument.

If time is measured with a stopwatch, time is the variable and the stopwatch is the instrument.

Step 4: Identify controlled variables

Ask:

What other factors could affect the result?

For the paper aeroplane investigation, possible controlled variables include:

• paper type
• overall design apart from wing length
• launch position
• launch method
• room or test area
• distance measurement method

You should keep these conditions as consistent as practical.

Control what could reasonably matter

Do not create a huge list of irrelevant objects.

Focus on factors that have a reasonable scientific connection to the measured outcome.

For example, launch method could affect flight distance.

The colour of a nearby chair probably would not, unless you had a particular scientific reason to investigate it.

Practice 2: Identify the variables

Question:

How does water temperature affect the time required for sugar to dissolve?

Independent variable:

Water temperature.

Dependent variable:

Time required for the sugar to dissolve.

Possible controlled variables:

• amount of water
• amount of sugar
• type of sugar
• container
• stirring method

Step 5: Decide exactly how the independent variable will change

Writing only different temperatures is not enough for a detailed plan.

You might instead plan to test:

• 20 degrees Celsius
• 30 degrees Celsius
• 40 degrees Celsius
• 50 degrees Celsius

The exact values depend on the question, available equipment and safety considerations.

Never choose conditions that create unnecessary risk.

Step 6: Decide exactly how the dependent variable will be measured

A good plan states:

• what will be measured
• which instrument will be used
• which unit will be recorded
• when the measurement begins
• when the measurement ends

For dissolving time, you could define the measurement as:

Start the stopwatch when the sugar enters the water and stop it when no visible sugar crystals remain.

This gives the procedure a clear measurement rule.

Operational definitions make vague ideas measurable

Sometimes the idea you want to investigate is too broad to measure directly.

For example:

Which conditions make a plant grow best?

You need to decide what grow best means in your investigation.

You might measure:

• increase in height
• number of new leaves
• change in mass

Your investigation should state which measurement represents the outcome you are studying.

Step 7: Choose suitable materials and equipment

List what the investigation requires before writing the procedure.

For a dissolving investigation, a materials list might include:

• water
• sugar
• identical containers
• measuring cylinder
• balance
• thermometer
• stopwatch
• stirring tool

The exact list depends on the method you choose.

Be precise where precision matters

Instead of:

some sugar

write:

5 grams of sugar for each trial

Instead of:

a cup of water

you might write:

100 millilitres of water

Precise quantities make a procedure easier to repeat.

Step 8: Plan safety before the procedure

Safety is part of experimental design, not something added after the plan is finished.

Ask:

• Could anything become too hot?
• Could glass break?
• Could something spill?
• Could an object move unexpectedly?
• Are electrical devices involved?
• Are chemicals involved?
• Are people, animals or biological materials involved?

Choose a safer method whenever a lower risk option can answer the same scientific question.

School investigations should follow teacher, laboratory and equipment safety instructions.

Know when an investigation needs supervision

Some questions are not suitable for an unsupervised home or classroom experiment.

Investigations involving dangerous chemicals, flames, high temperatures, electricity, harmful microorganisms, medicines, human participants or animals require appropriate rules and supervision.

A good scientist changes the plan when the original method creates unnecessary risk.

Step 9: Write a procedure another person can repeat

A scientific procedure should explain what happens in enough detail for someone else to understand the method.

Use numbered steps because order matters.

For example:

1. Measure 100 millilitres of water into the container.
2. Adjust the water to the first planned temperature.
3. Measure 5 grams of sugar.
4. Add the sugar to the water and start the stopwatch.
5. Stir using the planned method.
6. Stop the stopwatch when no visible sugar crystals remain.
7. Record the time in seconds.
8. Prepare the next trial using the same procedure.

Write what you will actually do

Avoid instructions such as:

Do the experiment carefully.

This does not tell another investigator what actions to perform.

Specific procedures reduce uncertainty.

Step 10: Explain how controlled variables will stay consistent

Do not merely list controlled variables.

Explain how you will control them.

For example:

Controlled variable How it will be controlled Amount of water Use 100 millilitres in every trial Amount of sugar Use 5 grams in every trial Container Use the same type and size of container Stirring method Use the same stirring pattern for every trial

This makes the plan much more useful.

Step 11: Plan repeated trials

One measurement may be affected by chance variation or an unnoticed mistake.

Repeating the investigation gives you more data and lets you see whether a pattern appears consistently.

Your procedure should state how many trials you plan for each condition.

For example:

Repeat each temperature condition three times.

The suitable number of trials depends on the investigation, available time and the type of measurement.

Repeating a trial is not the same as changing the condition

Suppose you test four temperatures and repeat each temperature three times.

You have four experimental conditions.

Each condition has three trials.

That gives twelve measurements in total.

Step 12: Plan the data table before collecting data

A useful data table can reveal missing parts of your plan before you begin.

For the dissolving investigation:

Water temperature Trial 1 time Trial 2 time Trial 3 time Mean time 20 degrees Celsius 30 degrees Celsius 40 degrees Celsius 50 degrees Celsius

The headings tell you what measurements must be collected.

Include units in your records

Numbers without units can be ambiguous.

Record:

42 seconds

rather than simply:

42

Use units consistently throughout the investigation.

Step 13: Decide how observations will be documented

Not all useful data are numerical.

You may also record observations such as:

• colour changes
• unexpected movement
• visible bubbles
• damage to a sample
• equipment problems
• changes made to the procedure

Record observations when they happen rather than relying on memory later.

Do not hide mistakes or unexpected events

If something goes wrong during a trial, record what happened.

For example:

Trial 2 was restarted because some water spilled before the sugar was added.

This information may matter when interpreting the results.

Step 14: Think about comparison before collecting data

Ask:

When I finish, how will I compare the conditions?

You might compare:

• individual measurements
• mean values from repeated trials
• changes over time
• patterns in a table
• patterns in a graph

You do not need to know the result in advance.

You only need a plan for how the evidence will be organized.

Do not design the test to force the answer you expect

A hypothesis is a prediction to be tested.

Your procedure should collect evidence whether the prediction turns out to be supported or not.

Unexpected evidence can still be useful scientific evidence.

Worked example: Paper aeroplane test

Question:

How does wing length affect the distance travelled by a paper aeroplane?

Independent variable:

Wing length.

Values:

8, 10, 12 and 14 centimetres.

Dependent variable:

Distance travelled from the launch line to the first point where the aeroplane touches the floor.

Controlled variables:

• paper type
• main design
• launch location
• launch method
• measurement method

Trials:

Three flights for each wing length.

Data:

Record each flight distance in metres and calculate the mean distance for each wing length.

Check the worked example

Ask whether another person could tell:

1. what is being changed
2. what is being measured
3. what should stay consistent
4. how many trials are planned
5. which unit will be used
6. how the conditions will be compared

If any answer is missing, the plan needs more detail.

Practice 3: Plan a ramp investigation

Question:

How does ramp height affect the distance travelled by a toy car after it leaves the ramp?

Write down:

• the independent variable
• three or four values you could test
• the dependent variable
• how you would measure it
• at least four controlled variables
• the number of trials

Possible plan

The independent variable is ramp height.

The dependent variable is distance travelled after the car leaves the ramp.

Possible controlled variables include the car, ramp surface, starting position, floor surface and measurement method.

The exact heights and number of trials should be chosen before testing.

Practice 4: Find the unfair comparison

A student wants to test whether water temperature affects dissolving time.

Trial A uses:

• 50 degree water
• 5 grams of sugar
• continuous stirring

Trial B uses:

• 20 degree water
• 10 grams of sugar
• no stirring

Why is this difficult to interpret?

Water temperature, amount of sugar and stirring method all changed.

A better design would vary temperature while keeping the amount of sugar and stirring method consistent.

Practice 5: Improve the measurement

Question:

Does exercise make the heart beat faster?

A vague dependent variable would be:

how fast the heart feels

A clearer measurement could be:

heart rate in beats per minute measured immediately after the planned exercise period

An investigation involving people would also require suitable safety and supervision.

Practice 6: Improve the procedure

Read:

1. Get some water.
2. Add sugar.
3. Stir it.
4. See how long it takes.

What information is missing?

Possible missing details include:

• amount of water
• water temperature
• amount of sugar
• container
• stirring method
• when timing begins
• when timing ends
• number of trials

Practice 7: Design the data table

Suppose you plan to test four ramp heights with three trials at each height.

Your table should make room for:

• ramp height
• trial 1 distance
• trial 2 distance
• trial 3 distance
• mean distance if appropriate

Include the measurement units in the headings.

Practice 8: Spot the missing controlled variable

A student tests how light duration affects seedling height.

The plants receive the same amount of water and grow for the same number of days.

However, different plant species are used in each light condition.

Why is this a problem?

Plant species could influence growth, so the comparison does not isolate light duration clearly.

Using the same species would improve the design.

Practice 9: Think about safety

A student proposes heating several containers of water to very high temperatures just to create a wider temperature range.

Ask:

Can the scientific question be answered using a safer range?

If yes, choose the safer range and follow appropriate supervision and equipment guidance.

Practice 10: Build your own investigation plan

Choose one simple question that can be tested safely.

Complete this planning structure.

Question

What do you want to find out?

Prediction

What result do you expect, and why?

Independent variable

What will you deliberately change?

Independent variable values

Which values or conditions will you compare?

Dependent variable

What will you measure?

Measurement method

How, when and in what unit will you measure it?

Controlled variables

Which important conditions will you keep consistent?

Materials

What equipment and quantities will you need?

Safety

What hazards or precautions must be considered?

Procedure

Write numbered instructions that another person could follow.

Trials

How many times will each condition be tested?

Data recording

Design the table before beginning.

Final review checklist

Before performing the investigation, check:

1. Is the question testable and measurable?
2. Is the independent variable clearly defined?
3. Are its planned values stated?
4. Is the dependent variable measurable?
5. Is the measurement method clear?
6. Have relevant controlled variables been identified?
7. Does the plan explain how they will remain consistent?
8. Are quantities and equipment stated clearly?
9. Is the procedure detailed enough to repeat?
10. Are repeated trials planned where appropriate?
11. Is the data table ready?
12. Have safety issues been considered?

What if your plan changes during the investigation?

Sometimes a practical problem appears only after testing begins.

If you must change the procedure, record the change.

Do not quietly alter the method and pretend every trial used the same procedure.

The change may affect how the results should be interpreted.

What if the hypothesis is not supported?

That does not automatically make the investigation unsuccessful.

The purpose of the experiment is to collect evidence that tests the prediction.

If the evidence does not support the prediction, you can examine the results, consider limitations and decide what could be investigated next.

The main idea

A fair scientific test begins with a clear and measurable question.

Change the factor you want to investigate, measure the response carefully and keep other important conditions as consistent as practical.

Plan the exact values, measurements, materials, safety precautions, procedure, repeated trials and data table before you begin.

A strong plan does not guarantee the result you expect.

It gives you a better chance of collecting evidence that can be understood and compared.

Continue learning

Review Independent, Dependent and Controlled Variables whenever you need help identifying the roles of variables in your investigation.

Sources and further reading

• University of Chicago, Select Your Variables
• University of Chicago, Write Your Procedure
• OpenStax, The Process of Science