Controlled experiments (AQA GCSE Statistics): Revision Notes
Controlled experiments
Scientists gather information through various experimental approaches to understand how the world works. When conducting research, they focus on investigating how changes in one factor (called the explanatory or independent variable) influence another factor (known as the response or dependent variable).
Understanding variables in experiments
In any scientific investigation, there are three important types of variables you need to understand:
Explanatory variable (Independent variable): This is the factor that the researcher deliberately changes or controls during the experiment. It's the 'cause' that scientists think might produce an effect.
Response variable (Dependent variable): This is what the researcher measures or observes to see if it changes. It's the 'effect' that might result from changes in the explanatory variable.
Extraneous variables: These are all the other factors that could potentially influence the results of an experiment, but which the researcher isn't particularly interested in studying. These variables must be carefully controlled or kept constant to ensure the results are meaningful and accurate.
Understanding the distinction between explanatory, response, and extraneous variables is fundamental to designing and interpreting scientific experiments. Every experiment involves all three types of variables, and successful researchers learn to identify and manage each type appropriately.
Ensuring reliable and valid results
When scientists repeat or replicate an experiment and get very similar results each time, this suggests that their data is both valid (measures what it claims to measure) and reliable (produces consistent results). This is why repetition is such a crucial part of good scientific practice.
The concepts of validity and reliability are essential for determining whether experimental results can be trusted. Without replication, it's impossible to know whether results occurred by chance or represent a genuine scientific finding.
What are controlled experiments?
Laboratory experiments take place in a controlled environment where scientists can carefully manage the conditions. This controlled setting is one of the key features that makes these experiments so valuable for research.
Example of a controlled experiment
Consider an investigation into how sunlight affects plant colour. In this experiment:
- Explanatory variable: The length of time plants are exposed to sunlight
- Response variable: The difference in colour of the plant leaves
- Plants would be kept in identical conditions except for the sunlight exposure time
Advantages of controlled experiments
Controlled experiments offer several important benefits. They're relatively easy to replicate, which means other scientists can repeat the work to verify results. Additionally, researchers can effectively control extraneous variables such as temperature, humidity, or other environmental factors that might otherwise interfere with the results.
The ability to control extraneous variables is what makes laboratory experiments so powerful for establishing cause-and-effect relationships. This control allows researchers to isolate the specific effect of their explanatory variable.
Disadvantages of controlled experiments
However, there are some limitations to consider. The main concern is that organisms or systems might behave differently in a controlled laboratory environment compared to their natural, real-life conditions. This means the results might not always accurately reflect what would happen in the real world.
The artificial nature of laboratory environments can be a significant limitation. Results that seem clear in controlled conditions may not translate directly to complex real-world situations where many variables cannot be controlled.
Worked example analysis
Worked Example: Analysing Sylvie's Sleep Medication Study
Let's examine a practical example: Sylvie wants to investigate whether a new type of medication helps people who have difficulty sleeping. She plans to conduct a laboratory experiment in a hospital with patients who suffer from this condition.
Step 1: Identifying the variables
The explanatory variable is the type of medication given to patients (this is what Sylvie is deliberately changing or controlling).
The response variable is the degree of difficulty the patient experiences when trying to sleep (this is what she's measuring to see if the medication has an effect).
Step 2: Analysing advantages of the laboratory approach
An advantage is that the medication dosage and timing can be precisely controlled, and the amount of sleep can be accurately measured using proper monitoring equipment in the hospital setting.
Step 3: Considering disadvantages of the laboratory approach
A disadvantage is that patients may find it more difficult to sleep naturally because they're in an unfamiliar hospital environment rather than their own home. This change in environment becomes an additional extraneous variable that's challenging to control in this laboratory setting.
Key Points to Remember:
- Controlled experiments take place in laboratory settings where extraneous variables can be carefully managed
- Explanatory variables are what researchers deliberately change, while response variables are what they measure
- Extraneous variables must be controlled to ensure reliable results
- Replication of experiments helps establish that data is valid and reliable
- Laboratory experiments are easy to repeat but may not reflect real-world conditions perfectly