Random assignment is used to control order effects in a within-subjects experiment.
Experimental Design: Types, Examples & MethodsBy Dr. Saul McLeod, updated 2017 Show
Experimental design refers to how participants are allocated to the different groups in an experiment. Types of design include repeated measures, independent groups, and matched pairs designs. Probably the commonest way to design an experiment in psychology is to divide the participants into two groups, the experimental group, and the control group, and then introduce a change to the experimental group and not the control group. The researcher must decide how he/she will allocate their sample to the different experimental groups. 2. Repeated measures /within-groups: The same participants take part in each condition of the independent variable. 3. Matched pairs: Each condition uses different participants, but they are matched in terms of important characteristics, e.g., gender, age, intelligence, etc. Animation created by Tom H Learning Check
1. In order to compare the effectiveness of two different types of therapy for depression, depressed patients were assigned to receive either cognitive therapy or behavior therapy for a 12-week period. The researchers attempted to ensure that the patients in the two groups had a similar severity of depressed symptoms by administering a standardized test of depression to each participant, then pairing them according to the severity of their symptoms. 2. To assess the difference in reading comprehension between 7 and 9-year-olds, a researcher recruited a group of each from a local primary school. They were given the same passage of text to read, and then asked a series of questions to assess their understanding. 3. To assess the effectiveness of two different ways of teaching reading, a group of 5-year-olds were recruited from a primary school. Their level of reading ability was assessed, and then they were taught using scheme one for 20 weeks. At the end of this period, their reading was reassessed, and a reading improvement score was calculated. They were then taught using scheme two for a further 20 weeks and another reading improvement score for this period was calculated. The reading improvement scores for each child were then compared. 4. In order to assess the effect of organization on recall, a researcher randomly assigned student volunteers to two conditions. Condition one attempted to recall a list of words that were organized into meaningful categories; condition two attempted to recall the same words, randomly grouped on the page. Experiment TerminologyEcological validityThe degree to which an investigation represents real-life experiences. Experimenter effectsThese are the ways that the experimenter can accidentally influence the participant through their appearance or behavior. Demand characteristicsThe clues in an experiment that lead the participants to think they know what the researcher is looking for (e.g. experimenter’s body language). Independent variable (IV)Variable the experimenter manipulates (i.e. changes) – assumed to have a direct effect on the dependent variable. Dependent variable (DV)Variable the experimenter measures. This is the outcome (i.e. result) of a study. Extraneous variables (EV)All variables, which are not the independent variable, but could affect the results (DV) of the experiment. Extraneous variables should be controlled where possible. Confounding variablesVariable(s) that have affected the results (DV), apart from the IV. A confounding variable could be an extraneous variable that has not been controlled. Random Allocation
Randomly allocating participants to independent variable conditions means that all participants should have an equal chance of taking part in each condition. The principle of random allocation is to avoid bias in the way the experiment is carried out and to limit the effects of participant variables. Order effectsChanges in participants’ performance due to their repeating the same or similar test more than once. Examples of order effects include: (i) practice effect: an improvement in performance on a task due to repetition, for example, because of familiarity with the task; (ii) fatigue effect: a decrease in performance of a task due to repetition, for example, because of boredom or tiredness. How to reference this article:How to reference this article:McLeod, S. A. (2017, January 14). Experimental design. Simply Psychology. www.simplypsychology.org/experimental-designs.html Home | About Us | Privacy Policy | Advertise | Contact Us Simply Psychology's content is for informational and educational purposes only. Our website is not intended to be a substitute for professional medical advice, diagnosis, or treatment. © Simply Scholar Ltd - All rights reserved Is random assignment used in withinThus, random assignment plays an important role in within-subjects designs just as in between-subjects designs. Here, instead of randomly assigning to conditions, they are randomly assigned to different orders of conditions.
What does random assignment control for in experimental research?Random assignment is a key part of experimental design. It helps you ensure that all groups are comparable at the start of a study: any differences between them are due to random factors.
What is random assignment and what does it control for?Random assignment refers to the use of chance procedures in psychology experiments to ensure that each participant has the same opportunity to be assigned to any given group. Study participants are randomly assigned to different groups, such as the experimental group or treatment group.
Is used to control order effects in an experiment?Counterbalancing is a technique used to deal with order effects when using a repeated measures design. With counterbalancing, the participant sample is divided in half, with one half completing the two conditions in one order and the other half completing the conditions in the reverse order.
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