Experiments (laboratory and field)

Q: What is the primary purpose of an experiment in psychological research?

The primary purpose of an experiment is to establish a cause-and-effect relationship by manipulating one variable to see if it causes a change in another.

Q: What is the difference between an Independent Variable (IV) and a Dependent Variable (DV)?

The Independent Variable (IV) is the factor manipulated by the researcher (the cause), while the Dependent Variable (DV) is the factor being measured (the effect).

Q: What does it mean to "operationalize" a variable?

Operationalization means defining a variable in precise, measurable terms so that the study can be replicated exactly.

Q: What are the key characteristics of a laboratory experiment?

A laboratory experiment takes place in a highly controlled, artificial environment where the researcher uses standardized procedures and minimizes extraneous variables.

Q: What is a major strength and a major weakness of laboratory experiments?

A strength is high reliability due to easy replication; a weakness is low ecological validity because the artificial setting may not reflect real-life behavior.

Q: How does a field experiment differ from a laboratory experiment?

A field experiment takes place in a natural environment (like a street or school) where the researcher still manipulates an IV but has less control over extraneous variables.

Q: What is an Independent Measures Design?

An Independent Measures Design is an experimental setup where different participants are used for each level of the Independent Variable.

Q: What are "order effects" and how are they managed in a Repeated Measures Design?

Order effects occur when participants' performance changes because they do multiple tasks (boredom or practice); they are managed using counterbalancing .

Q: What is a Matched Pairs Design?

A Matched Pairs Design involves pairing participants based on similar characteristics (like age or IQ) and then splitting each pair across the different levels of the IV.

Q: What is the difference between Internal and Ecological Validity?

Internal Validity is the extent to which the IV actually caused the change in the DV, while Ecological Validity is the extent to which findings apply to real-world settings.

Psychology (9990) · Cambridge International AS Level · 5 min read · Updated: 7 Sept 2026

Introduction to Experiments

Welcome to the heart of psychological research! If you have ever wondered, "Does caffeine actually help people focus?" or "Do children copy what they see on TV?", you are thinking like an experimental psychologist. The experiment is the only research method that allows us to find out a cause-and-effect relationship. In other words, we change one thing to see if it causes a change in another.

Don't worry if this seems like a lot of technical terms at first. Think of an experiment like a recipe: if you change the amount of sugar (the cause), you want to see if the cake tastes different (the effect). Let's dive in!

1. The Building Blocks: Variables

To run an experiment, we need two main types of variables. Variables are just things that can change or vary.

Independent Variable (IV)

The Independent Variable is the variable that the researcher manipulates or changes. It is the "cause." For example, in the study by Andrade (doodling), the \( IV \) was whether the participants doodled or did not doodle.

Dependent Variable (DV)

The Dependent Variable is the variable that the researcher measures. It "depends" on the \( IV \). In the Andrade study, the \( DV \) was the number of names and places remembered from a phone message.

Operational Definitions

In Psychology, we must be very specific. Operationalization means defining exactly how you will manipulate the \( IV \) and measure the \( DV \).
Example: Instead of saying "I will measure memory," you say "I will measure the number of words recalled from a list of 20."

Quick Review: The \( IV \) is the "Input" (what you change), and the \( DV \) is the "Data" (what you record).

2. Laboratory Experiments

A laboratory experiment takes place in an artificial, controlled environment. This doesn't always mean a room with white coats and test tubes; it just means a place where the researcher has high control over what happens.

Key Features:

High levels of standardisation (keeping everything the same for every participant).
High control over extraneous variables (outside things that might mess up the results).
Uses an experimental group (gets the treatment) and a control group (does not get the treatment, used for comparison).

Strengths:

Reliability: Because the procedure is standardised, it is easy for other researchers to replicate (repeat) the study to see if they get the same results.
Validity: Because we control outside distractions, we can be more certain that the \( IV \) really caused the change in the \( DV \).

Weaknesses:

Low Ecological Validity: The setting is artificial, so people might not behave the same way they would in real life.
Demand Characteristics: Participants might guess the aim of the study and change their behavior to "help" or "hinder" the researcher.

3. Field Experiments

A field experiment takes place in a natural environment (like a school, a street, or a hospital), but the researcher still manipulates an \( IV \).

Example from Syllabus: In the study by Piliavin et al. (subway Samaritans), the researchers manipulated the type of victim (ill vs. drunk) on a real moving subway train to see who would help.

Strengths:

High Ecological Validity: Participants are in their natural surroundings, so their behavior is more realistic.
Lower Demand Characteristics: Often, participants don't even know they are in a study, so they act naturally.

Weaknesses:

Less Control: It is harder to control extraneous variables (like how crowded the subway is).
Ethics: It can be hard to get informed consent if people don't know they are being studied.

4. Experimental Designs

This is the plan for how you use your participants. There are three types you need to know:

A. Independent Measures Design

Different participants are used in each level of the \( IV \).
Example: Group A doodles, Group B does not.

Pro: No order effects (participants don't get tired or bored).
Con: Participant variables (one group might just naturally have better memories than the other).
Fix: Use random allocation (flipping a coin to decide who goes in which group) to even out differences.

B. Repeated Measures Design

The same participants take part in every level of the \( IV \).
Example: Everyone doodles on Monday, and then everyone does not doodle on Tuesday.

Pro: No participant variables (you are comparing the person to themselves).
Con: Order effects. Participants might get better due to practice or worse due to fatigue (tiredness).
Fix: Use counterbalancing. Half do Level 1 then Level 2; the other half do Level 2 then Level 1. This "cancels out" the order effect.

C. Matched Pairs Design

Participants are matched into pairs based on a quality (like IQ or age). One person from the pair goes to Group A, the other to Group B.
Bandura et al. (aggression) used this by matching children on their pre-existing aggression levels.

Pro: Reduces participant variables and has no order effects.
Con: Very time-consuming and difficult to match people perfectly.

5. Ensuring Quality: Reliability and Validity

Reliability

Think of reliability as consistency. If you did the study again using the same standardised procedure, would you get the same results? Lab experiments are usually high in reliability.

Validity

Think of validity as accuracy. Are we actually measuring what we claim to measure?

Internal Validity: Did the \( IV \) cause the change in the \( DV \), or was it something else (an extraneous variable)?
Ecological Validity: Can the findings be applied to real-world settings?

6. Ethical Considerations

When running experiments, psychologists must follow rules to protect participants:

Valid Consent: Participants should agree to take part (and be "informed" of what will happen).
Minimising Harm: Participants should not be distressed or hurt.
Right to Withdraw: They can leave at any time.
Confidentiality: Their names and data must be kept private.
Debriefing: Explaining the true aim of the study after it's finished.

Summary: The "Cheat Sheet"

Laboratory Experiments: High control, low realism, easy to repeat. \( \uparrow \) Reliability, \( \downarrow \) Ecological Validity.

Field Experiments: Low control, high realism, harder to repeat. \( \downarrow \) Reliability, \( \uparrow \) Ecological Validity.

Independent Measures: Different people. (Watch out for participant variables!)

Repeated Measures: Same people. (Watch out for order effects!)

Top Tip for Paper 2: If you are asked to plan an experiment, always remember to operationalise your variables and explain how you will standardise the procedure to make it reliable!

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the primary purpose of an experiment in psychological research?

The primary purpose of an experiment is to establish a cause-and-effect relationship by manipulating one variable to see if it causes a change in another.

What is the difference between an Independent Variable (IV) and a Dependent Variable (DV)?

The Independent Variable (IV) is the factor manipulated by the researcher (the cause), while the Dependent Variable (DV) is the factor being measured (the effect).

What does it mean to "operationalize" a variable?

Operationalization means defining a variable in precise, measurable terms so that the study can be replicated exactly.

What are the key characteristics of a laboratory experiment?

A laboratory experiment takes place in a highly controlled, artificial environment where the researcher uses standardized procedures and minimizes extraneous variables.

What is a major strength and a major weakness of laboratory experiments?

A strength is high reliability due to easy replication; a weakness is low ecological validity because the artificial setting may not reflect real-life behavior.

How does a field experiment differ from a laboratory experiment?

A field experiment takes place in a natural environment (like a street or school) where the researcher still manipulates an IV but has less control over extraneous variables.

What is an Independent Measures Design?

An Independent Measures Design is an experimental setup where different participants are used for each level of the Independent Variable.

What are "order effects" and how are they managed in a Repeated Measures Design?

Order effects occur when participants' performance changes because they do multiple tasks (boredom or practice); they are managed using counterbalancing.

What is a Matched Pairs Design?

A Matched Pairs Design involves pairing participants based on similar characteristics (like age or IQ) and then splitting each pair across the different levels of the IV.

What is the difference between Internal and Ecological Validity?

Internal Validity is the extent to which the IV actually caused the change in the DV, while Ecological Validity is the extent to which findings apply to real-world settings.

What are the five key ethical considerations in psychological experiments?

The five key considerations are Valid Consent, Minimising Harm, Right to Withdraw, Confidentiality, and Debriefing.

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