Experimental Methods and Sampling
Research methods are the techniques psychologists use to investigate behaviour. Understanding these methods — and their strengths and limitations — is essential for evaluating studies throughout the course.
Types of Experiment
| Type | Description | Strengths | Weaknesses |
|---|
| Laboratory experiment | Conducted in a controlled environment; the researcher manipulates the IV and measures the DV | High control — can establish cause and effect; replicable | Artificial setting — low ecological validity; demand characteristics (participants may behave unnaturally) |
| Field experiment | Conducted in a real-world setting; the researcher still manipulates the IV | More natural behaviour — higher ecological validity | Less control over extraneous variables; harder to replicate; ethical issues (participants may not know they are being studied) |
| Natural experiment | The IV occurs naturally — the researcher does not manipulate it, only measures the DV | Can study variables that would be unethical to manipulate (e.g. the effects of deprivation) | Cannot establish cause and effect (no manipulation of IV); participant variables may confound results |
Key Terminology
| Term | Definition |
|---|
| Independent variable (IV) | The variable the researcher changes or manipulates |
| Dependent variable (DV) | The variable the researcher measures (the outcome) |
| Extraneous variable | Any variable other than the IV that could affect the DV — must be controlled |
| Confounding variable | An extraneous variable that has affected the DV, making results unreliable |
| Hypothesis | A testable prediction about the relationship between the IV and DV |
| Operationalisation | Defining variables in a way that can be measured (e.g. "memory" = "number of words correctly recalled from a list of 20") |
| Standardisation | Keeping the procedure the same for all participants to ensure fairness |
| Reliability | The consistency of results — would the same results be found if the study were repeated? |
| Validity | Whether the study measures what it claims to measure |
| Ecological validity | Whether the findings can be generalised to real-life situations |
Experimental Designs
| Design | How It Works | Strengths | Weaknesses |
|---|
| Independent groups | Different participants in each condition | No order effects; quick | Individual differences between groups may confound results; more participants needed |
| Repeated measures | Same participants in both conditions | Controls for individual differences; fewer participants needed | Order effects (fatigue, practice) — countered by counterbalancing |
| Matched pairs | Participants matched on key variables, then one from each pair is placed in each condition | Controls individual differences without order effects | Difficult and time-consuming to match; not perfect matches |
Sampling Methods
Sampling is how participants are selected from the target population (the group you want to generalise your findings to).
| Method | How It Works | Strengths | Weaknesses |
|---|
| Random sampling | Every member of the target population has an equal chance of being selected (e.g. names drawn from a hat, random number generator) | Unbiased — no researcher influence in selection; most representative | Difficult and time-consuming to achieve; still may not be perfectly representative by chance |
| Opportunity sampling | Selecting whoever is available at the time and place of the study | Easy, quick and convenient | Biased — sample may not be representative (e.g. all students from one school); researcher may unconsciously select certain people |
| Systematic sampling | Selecting every nth person from a list (e.g. every 5th name on a register) | Objective method — reduces bias; relatively simple | Still depends on how the list is ordered; may miss certain groups |
| Stratified sampling | Dividing the population into subgroups (strata) based on key characteristics (e.g. age, gender), then randomly selecting from each group in proportion | Highly representative of the population | Complex and time-consuming; requires detailed knowledge of the population |
Exam Tips
- Know the three types of experiment and when each is appropriate
- Be able to identify the IV and DV in any study description — this is tested frequently
- Understand the difference between extraneous and confounding variables
- Know all four sampling methods — strengths and weaknesses of each
- For AO3: when evaluating a study, comment on the sampling method, experimental design and type of experiment used
- Use the terms reliability, validity and ecological validity precisely