Thinking Like a Scientist
5. Experimental Design
Learning outcomes
- I can identify the steps involved in planning an investigation.
- I can design an investigation that answers a scientific question.
- I can select appropriate equipment and procedures.
- I can identify potential sources of error before conducting an experiment.
- I can explain how careful planning improves investigations.
Introduction
A successful scientific investigation does not begin in the laboratory—it begins with careful planning.
Before collecting any data, scientists decide what they want to investigate, how they will carry out the experiment, what equipment they need, and how they will ensure the investigation is fair and reliable.
Good experimental design helps scientists collect accurate data, reduce errors, and draw trustworthy conclusions.
What Is Experimental Design?
Experimental design is the process of planning a scientific investigation before it is carried out.
A well-designed experiment should:
- answer a scientific question,
- collect useful evidence,
- produce reliable results,
- minimize errors,
- be safe to perform.
Good planning increases the chances of obtaining meaningful results.
Step 1: Ask a Scientific Question
Every investigation begins with a clear, testable question.
Examples include:
- How does temperature affect the rate at which sugar dissolves?
- How does the amount of sunlight affect plant growth?
- Which material is the best thermal insulator?
A good question is:
- clear,
- specific,
- measurable,
- testable.
Step 2: Write a Hypothesis
Next, scientists write a hypothesis.
A hypothesis explains what they expect to happen and why.
Example:
If the temperature of the water increases,
then sugar will dissolve faster,
because warmer water particles move more quickly and collide with the sugar more often.
The hypothesis guides the investigation.
Step 3: Identify the Variables
Scientists identify three types of variables.
| Variable | Description |
|---|---|
| Independent Variable | The factor that is deliberately changed. |
| Dependent Variable | The factor that is measured or observed. |
| Controlled Variables | Factors that are kept the same to ensure a fair test. |
Identifying variables helps ensure that only one factor is being tested.
Step 4: Select Equipment
Scientists choose equipment that is:
- suitable,
- accurate,
- safe,
- appropriate for the investigation.
Examples include:
| Investigation | Equipment |
|---|---|
| Measuring length | Ruler, metre stick, vernier calipers |
| Measuring mass | Electronic balance |
| Measuring temperature | Thermometer |
| Measuring time | Stopwatch |
| Measuring liquid volume | Measuring cylinder |
Choosing the correct equipment improves the quality of the data collected.
Step 5: Plan the Procedure
A procedure is a series of step-by-step instructions describing how the investigation will be carried out.
A good procedure should:
- be clear,
- be logical,
- be repeatable,
- include safety precautions,
- describe how measurements will be recorded.
Another scientist should be able to repeat the investigation using the written procedure.
Step 6: Identify Possible Sources of Error
Before beginning the experiment, scientists think about what might affect the results.
Possible sources of error include:
- reading scales incorrectly,
- faulty equipment,
- changing environmental conditions,
- inconsistent timing,
- human mistakes.
Planning ahead allows scientists to reduce these problems.
Step 7: Plan How to Record Data
Scientists decide how the results will be organized.
They often prepare:
- data tables,
- observation sheets,
- graphs,
- diagrams.
Preparing these before the experiment saves time and reduces mistakes.
Example:
| Trial | Temperature (°C) | Time to Dissolve (s) |
|---|---|---|
| 1 | 20 | |
| 2 | 40 | |
| 3 | 60 |
Why Is Careful Planning Important?
Careful planning helps scientists:
- collect accurate data,
- avoid unnecessary mistakes,
- improve safety,
- repeat investigations,
- compare results,
- draw reliable conclusions.
Poor planning often leads to unreliable or unusable data.
Example Investigation
Scientific Question
How does water temperature affect the time taken for sugar to dissolve?
Independent Variable
Water temperature
Dependent Variable
Time taken for the sugar to dissolve
Controlled Variables
- Amount of sugar
- Volume of water
- Type of sugar
- Container size
- Stirring method
Equipment
- Beakers
- Thermometer
- Stopwatch
- Measuring cylinder
- Sugar
- Stirring rod
Procedure
- Measure equal volumes of water.
- Heat the water to different temperatures.
- Add the same amount of sugar to each beaker.
- Stir each sample using the same method.
- Measure the time taken for the sugar to dissolve.
- Record the results in a table.
- Repeat each trial several times and calculate the average.
Real-World Applications
Experimental design is important in many fields, including:
- Medical research
- Engineering
- Environmental science
- Agriculture
- Pharmaceutical development
- Product testing
- Space exploration
- Food science
Careful planning helps ensure that investigations produce reliable and useful results.
Worked Examples
Example 1
What is the first step in planning an investigation?
Answer:
Ask a clear, testable scientific question.
Example 2
Why should only one independent variable be changed?
Answer:
So that any changes in the dependent variable can be attributed to that one factor.
Example 3
Name two pieces of equipment that could be used to measure time and temperature.
Answer:
- Stopwatch
- Thermometer
Example 4
Give two possible sources of error in an experiment.
Answer:
Possible answers include:
- Reading a measuring scale incorrectly.
- Faulty or poorly calibrated equipment.
- Human reaction time.
- Changes in room temperature.
Example 5
Why should scientists repeat experiments?
Answer:
Repeating experiments improves the reliability of the results and helps identify unusual or inconsistent measurements.
Did You Know?
Before launching a spacecraft, engineers perform thousands of carefully planned tests on individual components and complete systems. Every experiment is designed in advance, repeated many times, and checked for possible sources of error. This careful experimental design helps ensure that missions succeed even after travelling millions of kilometres through space.
Key Terms
| Term | Definition |
|---|---|
| Experimental Design | The process of planning a scientific investigation before it is carried out. |
| Procedure | A sequence of step-by-step instructions used to perform an investigation. |
| Equipment | The tools and instruments used during an experiment. |
| Variable | A factor that can change during an investigation. |
| Source of Error | Anything that may reduce the accuracy or reliability of the results. |
| Data Table | A table used to organize and record observations or measurements. |
Key Takeaways
- Experimental design is the process of carefully planning a scientific investigation.
- Good investigations begin with a clear scientific question and a testable hypothesis.
- Scientists identify variables, choose suitable equipment, and develop a detailed procedure before collecting data.
- Thinking about possible sources of error helps improve the quality of the investigation.
- Preparing data tables and repeating trials improves the reliability of the results.
- Careful planning leads to safer experiments, more accurate measurements, and stronger scientific conclusions.