Science for Everybody

Mark value: 2 marks

AO focus: AO1

Bullet-point answer points

• The researcher manipulates the independent variable.
• The dependent variable is measured in a highly controlled environment.

Full model answer

A laboratory experiment is an experiment in which the researcher manipulates the independent variable and measures its effect on the dependent variable in a highly controlled environment.

Common pitfalls and how to avoid them

Do not define a laboratory only as a room containing scientific equipment. The key features are researcher manipulation of the IV and high control over the setting and extraneous variables.

Mark value: 4 marks

AO focus: AO1

Bullet-point answer points

• Laboratory experiments take place in a highly controlled environment, whereas field experiments take place in a natural everyday setting.
• Laboratory experiments usually allow greater control of extraneous variables, whereas field experiments usually have less control.
• Field experiments may have greater ecological validity because behaviour is more natural.
• Both can involve researcher manipulation of the IV, so both can be true experiments.

Full model answer

A laboratory experiment takes place in a highly controlled environment, whereas a field experiment takes place in a more natural everyday setting. As a result, laboratory experiments usually give the researcher greater control over extraneous variables, while field experiments often produce more natural behaviour and may have higher ecological validity. In both methods, the researcher can manipulate the IV.

Common pitfalls and how to avoid them

Do not say that field experiments have no IV manipulation. That would describe a natural experiment more closely. Make each difference comparative by referring to both methods.

Mark value: 4 marks

AO focus: AO2

Bullet-point answer points

• The IV is exposure to the new flight path before versus after the change.
• The IV changed because of an external real-world event.
• The psychologist did not manipulate or create the change in the IV.
• Stress level is the DV measured by the psychologist.

Full model answer

This is a natural experiment because the independent variable, exposure to the new airport flight path, changed naturally rather than being manipulated by the psychologist. The psychologist simply took advantage of the external change and measured its possible effect on residents’ stress levels, which are the dependent variable. Therefore, the defining feature is the naturally occurring IV.

Common pitfalls and how to avoid them

Do not classify the study as a field experiment simply because it takes place in a real community. Experiment type depends on how the IV is created, not only on the setting.

Mark value: 6 marks

AO focus: AO2 and AO3

Bullet-point answer points

• The quasi-IV is diagnosis: dyslexia versus no dyslexia diagnosis.
• Diagnosis is a pre-existing participant difference and is not manipulated by the researcher.
• Participants cannot be randomly allocated to having or not having dyslexia.
• A limitation is that the groups may differ in other ways, such as educational history, age or reading experience.
• These differences may confound the relationship between diagnosis and memory performance.
• Therefore, causal conclusions are weaker than in a tightly controlled true experiment.

Full model answer

The study is a quasi-experiment because the independent variable is a pre-existing difference between participants: whether they have a dyslexia diagnosis. The researcher cannot manipulate this characteristic or randomly allocate participants to have dyslexia. One limitation is that the two groups may differ in other ways, such as their educational experiences or previous reading practice. These variables could influence performance on the reading-memory task, so any difference in the dependent variable may not be caused by dyslexia alone. This reduces internal validity and weakens causal conclusions.

Common pitfalls and how to avoid them

Do not describe diagnosis as a manipulated IV. Apply the limitation directly to the scenario by identifying a plausible group difference that could affect the DV.

Mark value: 8 marks

AO focus: AO1 and AO3

Bullet-point answer points

• Both can involve direct researcher manipulation of the IV and measurement of a DV.
• Laboratory experiments use highly controlled settings; field experiments use natural settings.
• Laboratory experiments usually offer stronger control of extraneous variables and easier replication.
• This can increase internal validity and confidence in cause and effect.
• Field experiments often produce more natural behaviour and may have higher ecological validity.
• Participants in field settings may be less affected by demand characteristics.
• Field experiments may have greater ethical difficulties around consent and privacy.
• Field settings make precise replication and control of confounding variables more difficult.

Full model answer

Laboratory and field experiments are similar because the researcher can directly manipulate an independent variable and measure its effect on a dependent variable. This means both can be true experiments. However, a laboratory experiment takes place in a highly controlled environment, whereas a field experiment takes place in a natural everyday setting. The main advantage of the laboratory is control. Extraneous variables can be standardised or held constant, so the study often has higher internal validity and is easier to replicate. However, the controlled setting may feel artificial and participants may show demand characteristics, which can reduce ecological validity. In contrast, field experiments may produce more natural behaviour because participants are in a familiar environment and may not realise they are being studied. This can improve ecological validity. However, researchers have less control over extraneous variables, replication is more difficult, and informed consent or privacy may become ethical concerns. Therefore, laboratory experiments tend to prioritise control, while field experiments tend to prioritise realism.

Common pitfalls and how to avoid them

Avoid writing two separate descriptions with no comparison. Use linking phrases such as ‘whereas’, ‘in contrast’ and ‘both’. Do not assume that every field study is automatically high in validity; explain why the setting may improve realism.

Mark value: 12 marks

AO focus: AO1 and AO3

Bullet-point answer points

• Natural experiments use an IV that changes independently of the researcher.
• Quasi-experiments use a pre-existing participant difference as the IV.
• Neither method normally allows direct manipulation of the IV or random allocation to the defining condition.
• Both allow research into variables that would be unethical or impossible to manipulate.
• Natural experiments can examine important real-world events and may have good ecological validity.
• Quasi-experiments may be run under controlled conditions, allowing some standardisation of the DV measurement.
• Both are vulnerable to confounding variables and pre-existing group differences.
• This weakens internal validity and makes cause-and-effect conclusions less secure.
• Natural events may be rare and difficult to replicate, reducing reliability and research opportunities.
• Samples may be specialised or culturally specific, limiting population and cultural generalisation.
• Findings from older natural events may also have reduced temporal validity if social conditions have changed.
• Overall usefulness depends on whether practical and ethical access outweighs the loss of experimental control.

Full model answer

Natural and quasi-experiments are useful because they allow psychologists to investigate variables that cannot easily be manipulated. In a natural experiment, the IV changes because of an external event or circumstance, whereas in a quasi-experiment the IV is a pre-existing participant difference such as age or diagnosis. In both cases, the researcher measures the DV but does not directly create the defining IV conditions.

A major strength is practical and ethical access. Researchers cannot randomly assign people to many important life experiences or personal characteristics. Natural and quasi-experiments therefore make it possible to study questions that a laboratory experiment could not investigate ethically. Natural experiments may also examine behaviour in realistic circumstances, which can improve ecological validity. Quasi-experiments can sometimes be carried out under controlled conditions, allowing standardised measurement even though the IV itself is not manipulated.

However, the lack of random allocation creates a serious problem for internal validity. Participants in different conditions may already differ in ways that affect the DV. Similarly, a naturally occurring event may happen alongside other social or environmental changes. These confounding variables provide alternative explanations, so researchers should be cautious about claiming cause and effect.

There are also problems with replication and generalisation. A rare natural event may never occur in exactly the same way again, making precise replication difficult. Quasi-experiments may rely on small or specialised groups, which can create sample bias. Findings may also reflect one culture or historical period, reducing cultural or temporal validity. Overall, natural and quasi-experiments are valuable when direct manipulation is impossible, but their conclusions are usually less causally secure than those from well-controlled true experiments.

Common pitfalls and how to avoid them

Do not treat natural and quasi-experiments as identical. Define each method clearly before evaluating it. Avoid generic evaluation: explain why lack of random allocation, pre-existing differences and naturally occurring changes create confounds and weaken causal conclusions.