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examBoard: AQA
examType: GCSE
lessonTitle: Randomisation
Randomisation is a crucial technique in psychological research that helps scientists make their studies fair and reliable. When we randomly assign participants or conditions, we reduce the chance that our results are affected by things we didn't plan for.
Key Definitions:
Imagine you're testing a new memory technique, but you only use people who are already good at remembering things. Your results wouldn't tell us if the technique works for everyone! Randomisation helps make sure our groups are balanced and our findings are trustworthy.
Randomisation helps control for participant variables (like age, gender, or intelligence) that might affect results. It spreads these variables evenly across conditions, making it more likely that any differences we find are due to our experimental manipulation.
Psychologists use several different methods to achieve randomisation in their studies. Each has its own advantages and works best in different situations.
These are the most common ways researchers randomly assign participants to different groups:
Like flipping a coin or drawing names from a hat. Each participant has an equal chance of being in any group. Easy to do but can lead to uneven groups with small samples.
Participants are first divided into subgroups (like by gender or age), then randomly allocated from each subgroup. Ensures key characteristics are balanced across groups.
Participants are allocated in "blocks" to ensure equal numbers in each condition. Useful for smaller studies where simple randomisation might create imbalanced groups.
Researchers use various tools and methods to achieve true randomisation:
The famous Hawthorne studies in the 1920s and 1930s initially failed to use proper randomisation. Researchers were studying how workplace conditions affected productivity, but because they didn't randomly assign workers to different conditions, their results were affected by selection bias. Workers who knew they were being studied changed their behaviour (now called the "Hawthorne Effect"). This case demonstrates why randomisation is essential - without it, we can't be sure if our results are due to our experimental manipulation or other factors.
The way randomisation is applied depends on the type of study being conducted:
In true experiments, randomisation is used in two key ways:
Participants are randomly assigned to either the experimental or control group. This helps ensure that any pre-existing differences between participants (like intelligence, personality, or motivation) are evenly distributed across groups.
In within-subjects designs (where the same participants experience all conditions), the order of conditions is randomised to control for order effects like practice, fatigue, or boredom.
When testing the effectiveness of CBT for depression, researchers randomly allocate participants to either receive CBT or a control treatment (like standard care or a waiting list). This randomisation ensures that factors like severity of depression, age, gender and previous treatment history are distributed evenly between groups. Without randomisation, people with less severe depression might be more likely to volunteer for the experimental treatment, making CBT appear more effective than it really is.
If you're conducting your own psychological research, here are some practical tips for effective randomisation:
Even experienced researchers sometimes make these errors with randomisation:
Randomisation is one of the most powerful tools in a psychologist's toolkit. It helps ensure that our research findings are valid, reliable and not influenced by factors we didn't intend to study. By understanding and properly implementing randomisation techniques, researchers can design studies that produce trustworthy results and advance our understanding of human behaviour and mental processes.
Remember that randomisation is just one aspect of good research design - it works alongside other techniques like standardisation, counterbalancing and appropriate control conditions to create robust psychological studies.
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