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Experimental Designs

What is the best experimental design in psychology? When it comes to experimental designs, one size doesn't fit all, and choosing the right one for your research is crucial. A design that works great in one context won't necessarily be the appropriate choice for a different study. In this article, we'll go through experimental designs in psychology, consider the strengths and…

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Experimental Designs

Experimental Designs
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What is the best experimental design in psychology? When it comes to experimental designs, one size doesn't fit all, and choosing the right one for your research is crucial. A design that works great in one context won't necessarily be the appropriate choice for a different study. In this article, we'll go through experimental designs in psychology, consider the strengths and weaknesses of each one and consider what contexts they could be applied in.

  • First, we'll introduce the experimental design definition.
  • Next, we'll describe the three types of experimental design, specifically independent measures, repeated measures and matched-pairs design.
  • We'll also talk about how the quasi-experimental research design can be applied in research.
  • We'll follow by providing experimental design examples in practice.
  • Finally, we'll compare and evaluate the different types of experimental design psychology.

Experimental designs, hands holding puzzles, StudySmarterFig. 1 - It's important to consider what experimental design best fits your research question.

Experimental Design Definition

To investigate the effect of our independent variable on the dependent variable, we need to expose participants to experimental conditions. The independent variable is then manipulated between these conditions.

Let's say you want to measure sleep's impact on a student's academic performance. Your independent variable is the amount of sleep that students get. You need to expose your participants to at least two different levels of sleep to investigate its impact on their academic performance.

You decide you want participants to get only three hours (sleep-deprived experimental condition) of sleep in one condition and 8 in the second (well-rested experimental condition).

The question now should the same group be subjected to both levels of your independent variable? Or should you create two groups where each is assigned to only one condition? These are questions related to choosing your experimental design.

Experimental design refers to the ways participants are assigned to the different conditions of an experiment. Your experimental design can involve subjecting the same group of participants to all levels of your independent variable or just one level.

Types of Experimental Design

There are four types of experimental design: independent measures, repeated measures, matched pairs, and quasi-experimental designs.

Various factors determine the type of experimental design used, like:

  • The research method used.
  • Whether the researcher can manipulate the variables studied.
  • Whether participants can be randomly allocated into the experimental (or control) groups.
  • Characteristics of participants.

Let's now go through each experimental research design used in psychology.

Experimental Research Design: Independent Measures

The independent measures design is an experimental design in which you only assign participants to one of your experimental conditions. This experimental design is also known as the between-subjects design.

So, if you have two conditions (e.g. one involving getting three hours of sleep before an exam and one that involves getting eight hours), you will need two groups. Each of the participants taking part in your experiment would only be assigned to one of the conditions.

However, if you're comparing two different groups of people, you need to account for the potential individual differences between the groups to ensure your findings are valid. Randomly assigning participants to groups is one way to average out any between-group differences.

It might skew your results if your eight-hour sleep group is much more academically advanced than your three-hour sleep group. To control for this, you can allocate participants randomly to each of the two conditions.

Random assignment means that each participant has a 50% chance of being assigned to either group. When you have two conditions, this could be done by flipping a coin when assigning each participant to the group or using a random number generator.

Experimental designs, bar chart visualising the difference in exam performance between  two conditions (3 hours and 8 hours), StudySmarter, Fig. 2 - Hypothetical results from the experiment investigating the impact of sleep on exam performance.

Using the independent measures design, you can compare the two experimental groups in your experiment. This design also allows you to avoid order effects and demand characteristics. Order effects occur when the order in which participants participate in the condition affects their performance.

By only taking the exam once, participants don't have a chance to practice or get bored with the task, which could skew results if not considered.

Demand characteristics occur when participants guess the aim of your experiment and adjust their behaviour to what they think is expected.

Experimental Research Design: Repeated Measures

In a repeated measures design, participants take part and are assessed in each experimental condition. Therefore, the data for each IV condition come from the same participants. This experimental design is also referred to as a within-subjects design.

This approach eliminates the potential individual differences between groups, which is a confounding variable. This way, repeated measures design increases the study's validity.

Suppose the same group of students participate in our sleep and academic performance experiment conditions. In that case, we know our results won't be affected by differences in academic ability, intelligence, or motivation between the experimental groups.

It also makes recruitment easier, as we need half the number of participants that we would have needed for the independent measures design. However, this design introduces the risk of order effects as well as demand characteristics.

For example, participants may perform better in the second condition because they know the task already (practise effect) or may not perform as well due to fatigue (fatigue effect).

Moreover, if the participants know what conditions you're investigating, they can modify their behaviour to fit what they think you expect.

Here, if we made the same participant take the test after eight hours of sleep and then again after three hours, they might guess that we expect their performance to be lower with less sleep. Therefore, they can put less effort into the second condition.

To minimise order effects in repeated-measures experimental designs, we can counterbalance the order in which participants participate in the two conditions. Counterbalancing involves subjecting half of the participants to the first condition first and the other half to the second condition first. This way, it is possible to determine how order effects influenced the results.

Experimental Research Design: Matched-Pairs

Similarly to the independent-measures design, the matched-pairs design involves subjecting participants to only one experimental condition. However, in this design, the assignment process is more complex. Participants are first paired based on specific characteristics that could be potential confounding variables. Then each individual in the matched pair is randomly assigned to an experimental or control group.

In our experiment, we could first match the students we recruited based on their IQ and past academic performance. Let's say that Jess and Fiona performed similarly on both of these dimensions. We would flip a coin to decide which group Fiona would be assigned to and assign Jess to the other.

In some way, this design combines the best of both words – it allows us to minimise both the order effects and the individual differences. However, this design can be more complex, costly and time-consuming.

Finding two groups of participants that match all the key characteristics that might influence your results can be difficult.

The ideal participants for the matched-pairs design would be monozygotic twins, likely to be similar in many personality characteristics and share 100% of their genes.

Quasi-experimental Research Design

The quasi-experimental design is similar to the independent measures design, as it tests different participants at each independent variable level, except that participants are not randomly assigned to conditions. Instead, this design utilises naturally existing groups and investigates differences between them.

For example, to investigate the difference in prejudice between Irish and English. It would be impossible to manipulate someone's nationality, so we must use already existing groups.

Similarly, if we wanted to study the impact of economic status on prosocial behaviour, we would also utilise this design. It would be unethical to manipulate the participant's economic status.

The advantage of this design is that it has high external validity, as it involves testing participants in real-life settings. But it can introduce individual differences that could confound our results.

Since the independent variable is not manipulated in quasi-experimental designs, findings from studies using this design are correlational.

Experimental designs, people walking in the city with a white linear chart in the background, StudySmarterFig. 3 - Some variables like demographics, genes or personality cannot be manipulated. When investigating these variables, we need to rely on quasi-experimental designs.

Experimental Design Examples

Independent measures research example:

  • Investigating how the amount of sleep influences attention by recruiting 20 participants and randomly assigning half to each experimental condition (4 hours and 10 hours of sleep).

Repeated measures research example:

  • Investigating differences in depression scores before and after cognitive behavioural therapy treatment.

Matched-pairs research example:

  • Investigating the effect of exercise on weight loss, 50 participants were recruited. The participants were paired by BMI, age and gender, two males aged 21 with a BMI of 28, two females aged 22 with a BMI of 26, etc. These pairs were randomly assigned to the control group (no exercise) and the experimental group (2h of moderate-intensity exercise a week) to measure weight loss.

Quasi-experimental research example:

  • Investigating the effectiveness of CBT for participants with depression or schizophrenia diagnosis. Here, the naturally occurring independent variable is the psychiatric diagnosis.

Evaluation of Experimental Design Psychology

Let's compare the occurrence of potential confounding variables (order effects, demand characteristics and individual differences) in each experimental design type.

Experimental designOrder effects and demand characteristicsIndividual differences
Individual measuresnoyes
Repeated measuresyesno
Matched-pairsnono (controlled)
Quasi-experimentalnoyes

Experimental Designs - Key takeaways

  • Experimental design refers to how participants are assigned to the different conditions of an experiment.
  • There are four types of experimental designs: independent measures, repeated measures, matched pairs, and quasi-experimental designs.
  • Independent measures design involves assigning participants to only one of your experimental conditions. This can introduce confounding variables due to individual differences between groups, which can be controlled using random assignment.
  • Repeated measures design involves subjecting the same group of participants to all independent variable levels. This can introduce order effects that are controlled by counterbalancing the conditions.
  • Matched-pairs design involves pairing participants with specific characteristics that could be potential confounding variables. The quasi-experimental design utilises naturally existing groups and investigates differences between them.

Frequently Asked Questions about Experimental Designs

Experimental design refers to how participants are assigned to the different conditions of an experiment.

We must first define variables and hypotheses to identify an appropriate experimental design. We can then choose a suitable experimental design based on the following: 

  • Whether participants can be randomly allocated to control or experimental groups. 
  • If the same participants should be tested on each IV condition or if different participants should be used.
  • If participants can be matched based on key characteristics to understand more about the proposed hypotheses.

The quasi-experimental design tests different participants at each independent variable level, except that participants are not randomly assigned to conditions. The quasi-experimental design utilises naturally existing groups and investigates differences between them.

An example of an experimental design is a matched pairs design. This is when participants are paired in terms of specific characteristics such as age or ethnicity. 

Four types of experimental designs are: independent measures, repeated measures, matched pairs and quasi-experimental designs.

Final Experimental Designs Quiz

Experimental Designs Quiz - Teste dein Wissen

Question

What is an experimental design?

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Answer

Experimental design refers to how participants are assigned to the different conditions of an experiment. 

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What are four examples of experimental designs?

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The four experimental designs are independent measures, repeated measures, matched pairs and quasi-experimental designs.

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What is an independent measures design?

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Independent measures design involves assigning participants to only one of your experimental conditions. 

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What is a repeated measures design?

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Repeated measures design involves subjecting the same group of participants to all independent variable levels.

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What is a matched-pairs design? 

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Matched-pairs design involves pairing participants based on specific characteristics that could be potential confounding variables. 

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What is a quasi-experimental design?

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Answer

The quasi-experimental design tests different participants at each level of the independent variable. It utilises naturally existing groups and investigates differences between them.

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Question

For the following research scenario, ‘an investigation to identify if Clozapine (medication) is an effective drug to minimise symptoms of schizophrenia’, which experimental design would be appropriate to use and why?

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A quasi-experimental design would be appropriate because it can compare schizophrenia patients taking Clozapine (experimental group) and patients with schizophrenia using a different drug. This allows the researcher to identify if Clozapine or another drug is better at minimising symptoms of schizophrenia in the recruited sample. This design is required, as ethically, researchers cannot change patients medication because it may cause physical and/or psychological harm to participants.

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Question

How do independent group designs and quasi-experimental designs differ?

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Answer

They differ in terms of method of allocation. Independent group designs can allocate participants into groups (e.g. by using random allocation), but quasi-experimental designs can't allocate participants into groups, instead they use already existing groups.

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What is the advantage of using a repeated measure design over an independent experimental design?

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An advantage of using a repeated measures design instead of an independent design is that it controls for individual differences. 

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What are the advantages and disadvantages of using a matched-pairs experimental design?

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The advantage of using a matched-pairs design is that it accounts for individual differences and is not affected by order effects. The disadvantages are that it is challenging to match participants, and recruiting participants can be costly and time-consuming.

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What is the purpose of counterbalancing?

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Answer

To minimise order effects in repeated measures designs.

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What does counterbalancing involve?

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Counterbalancing involves subjecting half of the participants to the first condition first and the other half to the second condition first. 

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What is a repeated measures design?

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In the repeated measures, all participants experience all levels of independent variables.

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Why are repeated measures praised as cost-effective?

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Repeated measures design has great economic benefits as this design only requires half the number of participants with independent groups and matched pairs design. This is a tremendous economic benefit to the researchers as they will spend less time and resources to recruit participants.

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Why are repeated measures praised for having high validity?

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Participant variables are controlled because the same participants take part in both conditions. Participant variables are extraneous variables related to the individual characteristics of each participant and may influence their response. The same participants participate in each condition in a repeated measures design, so extraneous participant variables such as individual differences can be eliminated.

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How can research deal with the order effects?

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Counterbalancing is an experimental technique used to overcome order effects. Counterbalancing ensures each condition is tested equally first or second.

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How can research deal with the demand characteristics?

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A cover story about the test's purpose can prevent participants from guessing the research hypothesis. The cover story should be plausible but false. Researchers communicate this statement to participants to prevent the true hypothesis from being revealed. 

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What is a matched pairs design?

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Participants are paired up based on a specific trait or variables relevant to the study and then split into different conditions in the matched pairs design.

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Why don't matched pairs experiments have any issues with order effects?

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Matched pairs experiments don't have issues with order effects because all participants are only tested once in matched pairs. No practice effect or boredom effect will occur.

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Why is matched pairs design criticised as having low cost-effectiveness?

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Matched pairs design has a lower economic benefit as it requires more participants. Also, extra tests or criteria need to be formed to pair participants. 

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Why is matched pairs design praised for having reduced demand characteristics?

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Matched pairs experiments don't have issues with demand characteristics. This is because participants are only tested once in matched pairs. They are less likely to guess the aim of the experiment.

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Question

How can researchers deal with confounding participant variables?


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Through matched pairs design, researchers can manipulate the participant variables. Extraneous participant variables like individual differences cannot be eliminated but can be reduced, improving internal validity.

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How are participants allocated in the matched pairs design?

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In the matched pairs design, one participant is allocated into an experimental group and the other into a control; this assignment process should be random. 

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How would participants be matched in pairs?

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Participants are paired up in matched pairs based on a specific trait or variables relevant to the study. For example, by gender, IQ, age, ethnicity, and socioeconomic status.

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What are extraneous variables?

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External factors other than the independent variable that influence the dependent variable.

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What are the strengths of matched pairs designs?

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  • No order effects.
  • Reduced demand characteristics.
  • Better control of participant variables. 

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What are the weaknesses of matched pairs designs?

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It can be costly and time-consuming.

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In an independent group design...

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Different participants are used in each experimental condition. 

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Experimental designs measure the effect that...

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the independent variable has on the dependent variable.

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Which variable is manipulated in an independent group design?


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Answer

The independent variable is manipulated. 

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What is the dependent variable?

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The dependent variable is the variable which depends on the changes made to the independent variable. It is the 'effect' factor. 

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​How does independent group design differ from repeated group design? 


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Repeated group design uses the same individuals on two or more occasions for each experimental condition. Whereas independent group design uses different individuals.

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In order to fairly test the hypothesis, the groups must contain...

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the same number of individuals. 

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A key advantage to independent group design is that there are no order effects. What are order effects?

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Order effects refer to the order a material/ stimuli is presented to participants. As the experimental design relies on the same participants, if the same order is used in each condition, the participants may get bored (the boredom effect), which can affect their performance and lower the study's validity. 

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Why is requiring more participants in an independent group design an advantage? 


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It is an advantage because it increases the external validity of the results, making them more representative. 

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Why can requiring more participants compared to other experimental designs be an issue?

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It can be a disadvantage because it may mean that experimenting is more costly (i.e. requires more equipment). 

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What are participant variables?


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Participant variables are essential individual differences that are not accounted for that can affect the validity and reliability of findings.

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'Participant variables always effect the validity of the results' 

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True. 

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How can we counteract participant variables?

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By using random sampling. 

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What does random sampling mean?

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Random sampling means that each member of the population has an equal chance of being selected through random list generators on computers or placing names in a container and drawing them out randomly. 

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Why is random sampling important for experimental design?

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It is important because it encourages unbiased sampling and, therefore, unbiased results. 

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What is a hypothesis?

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A hypothesis is a suggestion based on limited evidence and a starting point for further investigation. 

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How many types of experimental designs are there? 

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Four.

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A study recruited 20 participants, ten of whom slept 4 hours, and ten slept for 12 hours. These participants were randomly assigned to their groups and took the same measures to assess attention span.


Which experimental design is this? 

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Answer

Independent measures.

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A study was interested in comparing the effectiveness of Cognitive Behavioural Therapy in people with depression or schizophrenia diagnosis.


Which experimental design was likely used in this study?

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Answer

Quasi-experimental.

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A study investigated the differences in depression scores before and after Cognitive Behavioural Therapy.


Which type of experimental design would be used in the study? 

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Answer

Repeated-measured.

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How do researchers determine the experimental design that will be used? 

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Answer

The experimental design depends on the study's hypothesis and variables, characteristics of the sample and potential confounds.

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Which of the following two experimental designs are pretty similar? 

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Independent measures

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How can individual differences be controlled in independent measures designs?

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They can be controlled by using random allocation.

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Which of the following experimental designs do the researchers have the least amount of control over how participants are assigned? 

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Answer

Quasi-experimental.

Show question

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