A good scientific investigation begins with a question that evidence can help answer.
In this lesson you will practise turning broad questions into testable questions, identifying the variables involved and writing a hypothesis and prediction that match the investigation.
Goal: turn a broad science question into a clear testable question, hypothesis and prediction.
Before you start
Read The Scientific Method Explained Step by Step before completing this lesson.
You do not need to perform a real experiment for this lesson. The activities focus on planning and scientific reasoning.
Start with something you wonder about
Scientific questions often begin with curiosity.
You might wonder:
- Why do some paper shapes fall more slowly than others?
- Does water temperature affect how quickly sugar dissolves?
- Does the amount of light affect plant growth?
- Does the length of a pendulum affect how quickly it swings?
These questions can lead to investigations, but some still need to become more specific before they can be tested clearly.
What makes a question testable?
A testable question is specific enough that observations or measurements can provide evidence related to the answer.
A useful beginner question often identifies:
- something that will be changed or compared
- something that will be measured or observed
- conditions that should remain reasonably consistent
For example:
Too broad
What makes a paper helicopter good?
The word good is unclear.
More testable
How does the wing length of the same paper helicopter design affect the time it takes to fall from the same height?
This question identifies something to change and something to measure.
Step 1: Identify what you will change
The factor deliberately changed in an investigation is often called the independent variable.
In the paper helicopter question, the independent variable is:
wing length
You could compare several wing lengths while keeping the basic helicopter design similar.
Step 2: Identify what you will measure
The result you measure or observe in response to the change is often called the dependent variable.
For the paper helicopter investigation, the dependent variable is:
fall time
You could measure the time in seconds.
Why is it called dependent?
The dependent variable is the result you are examining to see whether it changes when the independent variable changes.
In this example, you are asking whether fall time depends on wing length.
Step 3: Decide what should stay the same
Other important conditions should be kept as similar as reasonably possible so that the investigation focuses on the variable you want to study.
These are often called controlled variables.
For the paper helicopter investigation, you might keep the following the same:
- paper type
- basic helicopter shape
- drop height
- testing location
- timing method
- release method
If many important conditions change at the same time, it becomes harder to understand what caused a difference in the results.
Controlled variable and control group are different ideas
A controlled variable is a condition that you try to keep the same.
A control group or comparison condition is something used as a reference in certain kinds of experiments.
Not every beginner investigation requires a separate control group, but controlled variables are useful whenever changing several conditions could make the result difficult to interpret.
Step 4: Rewrite the question clearly
A useful question pattern is:
How does changing ___ affect ___?
The first blank can describe the independent variable.
The second blank can describe the dependent variable.
For example:
How does changing paper helicopter wing length affect fall time?
This is much easier to investigate than:
Which paper helicopter is best?
A testable question needs a measurable result
Consider:
Does music make plants happier?
The word happier is difficult to measure scientifically in this situation.
You could instead ask something measurable, such as:
Under the planned conditions, does exposure to a particular sound condition affect the average height of the same type of plant over a defined period?
The revised version specifies a result that can be measured.
It still requires a careful experimental plan before any conclusion could be justified.
Step 5: Write a hypothesis
A hypothesis is a proposed explanation or answer that can be examined using evidence.
For the paper helicopter investigation, you might write:
Changing the wing length of a paper helicopter will affect its fall time because the wings interact with the surrounding air as the helicopter falls.
This hypothesis connects the variable to a possible explanation.
The investigation can then gather evidence related to the idea.
A hypothesis does not need to be correct
The purpose of an investigation is not to protect your hypothesis.
The evidence may support it, fail to support it or suggest that the explanation should be revised.
A hypothesis that is not supported can still be scientifically useful because it helps narrow the possible explanations and can lead to better questions.
Step 6: Write a prediction
A prediction states what result you expect to observe if the hypothesis is useful.
A common beginner format is:
If ___ changes, then ___ will change because ___.
For example:
If paper helicopter wing length is increased while the other important conditions are kept similar, then the average fall time will increase because the larger wings will interact with more air during the fall.
This prediction gives the investigation a result that can be compared with measurements.
Hypothesis and prediction are related but not identical
The hypothesis proposes an explanation.
The prediction states an expected observable result.
For example:
Hypothesis
Wing length affects paper helicopter fall time because changing the wing shape changes how the helicopter interacts with the air.
Prediction
If wing length increases, then average fall time will increase under the planned testing conditions.
The prediction can be checked against data.
Do not force every hypothesis into the same sentence pattern
The familiar if and then pattern can help beginners write a clear prediction.
However, it is not the only way scientists express hypotheses or predictions.
The important features are that the idea is clear, connected to the question and capable of being examined with evidence.
Example 1: Water temperature and dissolving
Broad question:
What makes sugar dissolve faster?
More specific question:
How does water temperature affect the time required for the same amount of sugar to dissolve under the same stirring conditions?
Independent variable:
water temperature
Dependent variable:
time required for the sugar to dissolve
Controlled variables could include:
- amount of water
- amount of sugar
- type of sugar
- container
- stirring method
Possible hypothesis:
Water temperature affects how quickly sugar dissolves because temperature affects the movement and interactions of particles in the mixture.
Possible prediction:
If the water temperature is increased under the planned conditions, then the same amount of sugar will take less time to dissolve.
Example 2: Ramp height and toy car distance
Question:
How does the starting height of a toy car ramp affect the distance the car travels after leaving the ramp?
Independent variable:
starting height of the ramp
Dependent variable:
distance travelled after leaving the ramp
Possible controlled variables:
- same toy car
- same ramp surface
- same floor surface
- same starting method
- same measuring method
A hypothesis could propose that changing ramp height changes the car's motion.
A prediction could state the direction of the expected distance change.
Example 3: A question that is difficult to test
Consider:
Which animal is the most interesting?
This depends heavily on personal opinion.
Science can investigate measurable questions about animals, but there is no single scientific measurement for most interesting.
A different question might examine a measurable behaviour or physical characteristic instead.
Practice 1: Which question is more testable?
Choose the stronger scientific question.
Question A
Are tall plants better?
Question B
How does the number of hours of light affect the average height of the same type of plant over four weeks?
Answer
Question B is more testable because it identifies a condition to compare and a measurable result.
Practice 2: Find the independent variable
Question:
How does water temperature affect the time needed for a tablet to dissolve?
What is deliberately changed?
Answer
Water temperature.
That is the independent variable.
Practice 3: Find the dependent variable
Use the same question:
How does water temperature affect the time needed for a tablet to dissolve?
What is measured?
Answer
The time needed for the tablet to dissolve.
That is the dependent variable.
Practice 4: Find the controlled variables
For the dissolving investigation, which conditions should you try to keep the same?
Possible answers include:
- same type of tablet
- same amount of water
- same container type
- same measuring method
- same stirring condition
The exact controls depend on the final investigation design.
Practice 5: Improve the question
Broad question:
What makes a ball bounce well?
Rewrite it as a measurable question.
One possible answer
How does the drop height of the same ball affect its first bounce height on the same surface?
Now both the changed condition and measured result are clear.
Practice 6: Hypothesis or prediction?
Statement:
If the drop height increases, then the first bounce height will increase.
This is mainly a prediction because it states the expected result.
A hypothesis could also explain why the relationship is expected.
Practice 7: Find the problem
Question:
Does changing the paper type, wing length, drop height and helicopter mass affect fall time?
This changes many important factors at once.
It would be difficult to know which change was responsible for the result.
A better beginner investigation would focus on one main independent variable while keeping other important conditions similar.
Activity: Turn a broad question into a scientific plan
Choose one broad question:
- What makes a paper airplane fly farther?
- What changes how quickly something cools?
- What makes a toy car travel farther?
- What changes how quickly something dissolves?
Then complete these steps:
- Choose one factor to change.
- Choose one result to measure.
- Write a testable question.
- Identify the independent variable.
- Identify the dependent variable.
- List important controlled variables.
- Write a hypothesis.
- Write a prediction.
This is a planning activity. Do not perform an experiment unless the activity is safe and approved by your teacher or responsible adult.
A worked planning example
Broad idea:
What changes paper airplane distance?
Testable question:
How does adding one paper clip to the nose of the same paper airplane design affect average flight distance?
Independent variable:
paper clip condition
Dependent variable:
flight distance
Important controlled variables:
- same airplane design
- same paper type
- same starting position
- same testing area
- same measuring method
Hypothesis:
Adding a paper clip to the nose will affect flight distance because it changes how mass is distributed in the airplane.
Prediction:
If one paper clip is added to the nose under the planned conditions, then the average flight distance will differ from the distance without the paper clip.
Notice that the prediction does not need to pretend we already know the result.
Make sure the result can actually be measured
Compare:
Difficult to measure
Does the new design make the airplane nicer?
Measurable
Does the new design change average flight distance?
The second question identifies a quantity that can be recorded.
Do not write the conclusion before the investigation
A hypothesis and prediction are written before examining the final evidence.
A conclusion comes later.
The conclusion should explain what the actual observations and measurements support.
Do not change your hypothesis after seeing the results just to make it appear correct.
Instead, report what happened and explain whether the evidence supported the original idea.
A surprising result can be useful
Suppose your prediction says longer paper helicopter wings will increase fall time.
Your measurements show almost no difference.
That does not make the investigation useless.
You can ask:
- Was the wing length difference large enough?
- Was the timing method precise enough?
- Were enough trials completed?
- Did another uncontrolled condition affect the results?
- Should the hypothesis be revised?
Unexpected evidence can lead to a better investigation.
Scientific questions should also be safe and ethical
A question can be scientifically interesting but unsuitable for a beginner experiment.
Do not design investigations involving dangerous chemicals, fire, electrical hazards, medicines, harmful microorganisms, deliberate injury, unsafe consumption or harm to people or animals.
Choose safe classroom or household materials and follow instructions from a teacher or responsible adult.
A testable question checklist
Ask:
- Is the question specific?
- Can observations or measurements help answer it?
- Have I identified what will change?
- Have I identified what will be measured?
- Can important other conditions be kept reasonably consistent?
- Is the investigation safe and appropriate?
A hypothesis checklist
Ask:
- Does the hypothesis address the question?
- Does it propose an explanation or relationship?
- Can evidence from the investigation support or fail to support it?
- Have I avoided writing it as if the answer is already proven?
A prediction checklist
Ask:
- Does the prediction state an expected observable result?
- Does it connect the independent variable to the dependent variable?
- Could the result be compared with actual data?
Common mistakes to avoid
- asking a question based only on personal opinion
- using vague words such as better without defining what they mean
- changing several important variables at once
- choosing a result that cannot be observed or measured
- treating a hypothesis as a fact
- confusing a hypothesis with a prediction
- writing the conclusion before collecting evidence
- designing an unsafe investigation
Self check
- What makes a scientific question testable?
- What is the independent variable?
- What is the dependent variable?
- Why are controlled variables useful?
- What is the difference between a hypothesis and a prediction?
Suggested answers
- Evidence from observations or measurements can help answer it.
- The factor deliberately changed or compared in the investigation.
- The result that is measured or observed in response.
- They help keep the investigation focused on the relationship being tested.
- A hypothesis proposes an explanation or relationship, while a prediction states the expected observable result.
Lesson summary
Begin with a clear question that evidence can help answer.
Identify the independent variable, dependent variable and important conditions that should remain similar.
Write a hypothesis that proposes an explanation or relationship.
Then write a prediction describing the result you expect to observe.
The purpose of the investigation is to examine the idea with evidence, not to force the evidence to match the hypothesis.
Continue learning
Review The Scientific Method Explained Step by Step if you want to revisit how hypotheses fit into a complete investigation.
Review Observation and Inference: What Is the Difference? to practise separating evidence from interpretation.