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Experimental Design in Psychology

Design Context

What is Experimental Design?

Experimental design refers to how participants are allocated to conditions in an experiment. The design choice affects control over confounding variables, statistical analysis, and the validity of conclusions.

Key Experimental Designs

1

Independent Groups Design

Participants are assigned to only one condition.
2

Repeated Measures Design

The same participants complete all conditions.
3

Matched Pairs Design

Participants are matched across conditions based on key variables.

Independent Groups Design

✅
AdvantagesNo order effects or practice effects; simpler for participants.
❌
DisadvantagesMore participants needed; individual differences between groups can affect results, potentially increasing error variance.

Repeated Measures Design

✅
AdvantagesControls for individual differences entirely since each participant serves as their own control; fewer participants needed.
❌
DisadvantagesPotential for order effects such as practice, fatigue, or boredom affecting results; possible demand characteristics.

Control Measure

💡

Counterbalancing: Can be used to minimise order effects, where different participants experience conditions in different sequences.

Matched Pairs Design Summary

👥
It controls for some participant variables without the order effects of repeated measures.
⏳
However, it is time-consuming to match participants, and matching may be imperfect.

Key Terminology

🔄

Order Effects

Changes due to sequence (practice/fatigue).
❓

Demand Charac.

Participants guessing the aim and changing behavior.
⚠️

Confounding Var.

Variable affecting IV and DV, distorting results.
💪

Statistical Power

Ability to detect a true effect if one exists.

Factors in Choosing a Design

Research Consideration Impact
Practicality (availability of participants) High
Risk of confounding variables Critical
Risk of order effects Medium
Ethical implications (exposure to all conditions) High
Statistical power and precision Essential

Conclusion

Well-chosen experimental designs enhance internal validity and the reliability of the conclusions drawn from the data. Researchers weigh considerations such as practicality and ethical implications before selection.
Experimental Design Deck
Term
Experimental Design

What is experimental design?

Answer
Definition

The method of allocating participants to conditions in an experiment, affecting control, analysis, and validity.

Term
Independent Groups Design

What is an independent groups design?

Answer
Definition

Participants are randomly assigned to separate groups, each experiencing a different level of the independent variable.

Term
Key Advantage of Independent Groups

What is a key advantage of independent groups design?

Answer
Advantage

No order or practice effects.

Term
Main Disadvantage of Independent Groups

What is a main disadvantage of independent groups design?

Answer
Disadvantage

Requires more participants and may increase error variance due to individual differences.

Term
Repeated Measures Design

What is a repeated measures design?

Answer
Definition

The same participants experience all conditions of the experiment.

Term
Advantage of Repeated Measures

What advantage does repeated measures design offer?

Answer
Advantage

Controls for individual differences completely; fewer participants needed.

Term
Major Disadvantage of Repeated Measures

What is a major disadvantage of repeated measures design?

Answer
Disadvantage

Risk of order effects like practice, fatigue, or boredom.

Term
Controlling Order Effects

How can order effects be controlled in repeated measures?

Answer
Method

By using counterbalancing, where participants experience conditions in different sequences.

Term
Matched Pairs Design

What is matched pairs design?

Answer
Definition

Participants are matched on key variables across different conditions to make groups comparable.

Term
Disadvantage of Matched Pairs

What is a disadvantage of matched pairs design?

Answer
Disadvantage

Time-consuming matching process and possible imperfect matches.

Term
Factors When Choosing Design

What are factors to consider when choosing an experimental design?

Answer
Factors

Practicality, confounding variables, order effects, ethical issues, and statistical power.

Term
Importance of Right Design

Why is choosing the right experimental design important?

Answer
Importance

It improves internal validity and reliability of conclusions.

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🌸 Experimental Design Quiz

1. Which design assigns different participants to each condition?

Independent groups design involves random assignment of different participants to each condition.

2. What is an advantage of using repeated measures design?

Because the same participants take part in all conditions, fewer participants are needed overall.

3. Which method helps reduce order effects in repeated measures design?

Counterbalancing changes the order of conditions for different participants to minimize order effects.

4. A disadvantage of matched pairs design is:

Matching participants can be difficult and may not perfectly control all variables.

5. When choosing an experimental design, which factor is NOT usually considered?

While ethics and confounding are considered, participant preference for groups is generally not a key factor in design choice.

📊 Results

  • Notes and Quiz
  • Psychology
  • AS & A Level
  • Research Methods
  • WJEC