Controlling extraneous variables (Edexcel GCSE Statistics): Revision Notes
Extraneous variables
What are extraneous variables?
Extraneous variables are factors in an experiment that you're not specifically studying, but which could still influence your results and make them less reliable. Think of them as unwanted "background noise" that can interfere with what you're actually trying to measure.
For instance, imagine you're investigating whether background music helps factory workers be more productive. The music is what you want to test, but other things like how tired the workers are or their general health could also affect productivity. These other factors are extraneous variables because they're not part of your main investigation, yet they could change your results.
The key challenge with extraneous variables is that they can create false relationships in your data - making it appear that your treatment is working when it's actually something else causing the change, or hiding a real effect that your treatment is having.
Why controlling extraneous variables matters
The key reason for identifying and controlling extraneous variables is to make sure your results genuinely show the relationship between the variables you're actually interested in studying. If you don't control these unwanted variables, you can't be confident that any changes in your results are due to what you're testing, rather than these other interfering factors.
Worked example: memory test experiment
Worked Example: Identifying and Controlling Extraneous Variables
The experiment: Students took a memory test to see how many words from a list they could remember. On Day 1, they took the test in the main school hall. On Day 2, the same test was taken in the main hall again, but this time there was building work happening outside.
Breaking down the variables:
- Explanatory variable (what we're testing): The list of words given to students
- Response variable (what we're measuring): The number of words students remembered
- Extraneous variable (unwanted interference): The noise from building work outside
How to control it: Students could wear noise-cancelling headphones during the tests to eliminate the distraction from construction sounds.
Common extraneous variables and control methods
Here are the most frequent types of extraneous variables you'll encounter in experiments, along with practical ways to control them:
Noise and distractions
Examples: Building work, traffic, conversations, or any sounds that could disturb participants during an experiment. Control method: Use noise-cancelling headphones or conduct experiments in soundproofed rooms.
Temperature
Examples: A room being too hot or cold, lack of air conditioning on a warm day, or heating not working properly. Control method: Use fans, open doors and windows, or ensure consistent climate control throughout the experiment.
Environmental factors like temperature and lighting might seem minor, but they can significantly impact participant comfort and concentration, which directly affects their performance in your experiment.
Location
Examples: Conducting experiments in unfamiliar surroundings where participants might feel uncomfortable or distracted. Control method: Allow participants to complete the experiment in their own homes, or provide feedback sessions to help them feel more comfortable with the environment.
Lighting
Examples: Poor lighting that makes it difficult to read instructions or see materials clearly, or inconsistent lighting between different participants. Control method: Install additional lighting or ensure all participants have identical, adequate lighting conditions.
Practice question walkthrough
Practice Scenario: Sleep Medication Study
Scenario: A researcher investigates a new medication to help people sleep better. Patients stay overnight in a hospital where their heart rates are monitored throughout the night.
Questions to consider:
- What's the explanatory variable? (The new sleep medication)
- What's the response variable? (Heart rate measurements or sleep quality indicators)
- What extraneous variables might affect the results? (Hospital environment being unfamiliar, noise from other patients or medical equipment, different bed comfort levels, anxiety about being monitored, room temperature, etc.)
Exam tips
Critical Exam Strategy
When identifying extraneous variables in exam questions:
- Think about anything that could realistically affect the outcome but isn't part of the main investigation
- Consider the physical environment, participant characteristics, and external conditions
- Always suggest practical, realistic control methods
- Remember that good experimental design aims to keep everything constant except the variable being tested
Remember!
Key Points to Remember:
- Extraneous variables are unwanted factors that can affect your experimental results but aren't what you're actually investigating
- Common types include noise, temperature, location, and lighting conditions
- The goal is to control these variables so your results only reflect the relationship between your explanatory and response variables
- Always think practically about what could realistically interfere with an experiment in real-world conditions
- Good control methods are specific, practical, and achievable within the experimental setup