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In previous articles, we discussed how to organise your ideal experiment down to the last detail, such as objectives, hypotheses, sampling methods, pilots, and variables. In this one, we're getting down to business – the experimental designs.
Experimental design is the method in psychology by which participants (p) are assigned to different groups/conditions of an experiment. There are four types of experimental design: independent measurements, repeated measures, matched pairs, and quasi-experimental designs.
The independent measures design is an experimental design in which different p’s are used to test each condition of the independent variable (IV). This experimental design is also known as the between-subjects design. Participants are randomly assigned to experimental groups, i.e., they are assigned to a condition under which they will be studied/observed.
An example of a research scenario using an independent measures design is as follows: ‘A study recruited 20 participants, 10 of whom slept 4 hours and 10 other different p’s slept 12 hours. These P’s were randomly assigned to their groups and took the same measures to assess attention span (only the IV differed – the number of hours slept).
In a repeated measures design, p’s are assessed in each IV condition. Therefore, the data obtained for each IV condition come from the same participants. This experimental design is also referred to as a within-subjects design.
This experimental design may be considered problematic due to order effects, i.e., when the order of the presented conditions affects p’s performance. For example, p’s may perform better in the second condition because they know how to perform (practise effect) or may not perform as well due to fatigue (fatigue effect). Counterbalancing methods are used to counteract this. Half of the p’s complete the first condition first, and the others satisfy the second condition. In this way, it is possible to determine whether sequence effects influence the results.
An example of a research scenario using a repeated measures design is to investigate differences in depression scores before and after cognitive behavioural therapy.
In a matched-pairs design, p’s are paired concerning specific characteristics such as age, ethnicity, or education level. Each individual in the matched pair is randomly assigned to an experimental or control group.
An example of a study using a matched-pairs design is when 50 participants were recruited. The p’s were paired by age and gender, e.g., two males aged 21, two males aged 22, two females aged 21, two females aged 22, etc. These pairs were randomly assigned to the control group (standard diet) and the experimental group (keto diet) to measure weight loss.
The quasi-experimental design is similar to the independent measures design, except that p’s are not randomly assigned to a treatment or control group. Therefore, this method tests different p’s at each level of the IV. This method is typically used in psychological research to determine the effectiveness of an existing treatment/intervention.
Comparing the effectiveness of CBT in ‘healthy’ p’s and people with schizophrenia: as p’s with schizophrenia cannot be in the control group, we must use the quasi-experimental design as this research design does not allow random assignment to control and experimental groups.
Experimental design is the method in psychology by which participants (p) are assigned to different groups/conditions of an experiment.
To identify an appropriate experimental design, we must define variables and hypotheses. We can then choose a suitable experimental design based on:
The quasi-experimental design involves different participants being tested on each condition of the independent variable (IV). Participants are not randomly assigned to control and experimental groups for this design method.
An example of an experimental design is matched pairs design. This is when participants are paired in terms of specific characteristics such as age or ethnicity. Each individual of the matched pair is randomly assigned to an experimental or control group. An example of research that uses a matched-pair design is research that has recruited 50 participants that were paired in terms of age and gender, such as two males aged 21, two males aged 22, two females aged 21, two females aged 22, etc. These pairs were randomly assigned to the control group (standard diet) and the experimental group (keto diet) to measure weight loss.
Four types of experimental designs are: independent measures, repeated measures, matched pairs and quasi-experimental designs.
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