Strategies for random assignment in experimental designs
Experimental designs are crucial in psychology research. They allow psychologists to test the effectiveness of various psychological interventions. However, designing an effective experiment is not always easy. One crucial aspect of experimental design is the random assignment of participants to different groups. Random assignment ensures that the groups are similar in terms of their characteristics, and any observed differences are due to the intervention being tested and not any pre-existing differences between groups.
There are multiple strategies for random assignment. Some of these strategies are simple, while others are more complex. In this article, we will explore some of the most commonly used strategies for random assignment.
Simple random assignment
Simple random assignment is the most straightforward and the most commonly used method of random assignment. Each participant is assigned randomly to one of the experimental groups. This can be done by drawing names out of a hat, assigning numbers to participants and using a random number generator, or any similar method. The key is that each participant has an equal chance of being assigned to any group.
There are two important considerations when using simple random assignment. First, it is essential to ensure that the sample size in each group is reasonably similar. Second, it is crucial to monitor the randomization process to ensure that there is no bias.
Block randomization
Block randomization is a useful method when the sample size is small, and there is a high risk of bias. In block randomization, participants are randomized in smaller groups or blocks to ensure that each group has similar characteristics. For example, if the study includes 24 participants, they could be divided into four groups of six participants each. Participants are then randomly assigned to one of the four groups within each block.
One benefit of block randomization is that it minimizes the possibility of an unequal distribution of important variables, such as age or gender, between groups. When using block randomization, it is essential to ensure that the block size is a multiple of the number of groups. For example, if the number of groups is four, then block sizes of four, eight, or 12 are appropriate.
Stratified random assignment
Stratified random assignment is used when there are subgroups within the sample that may be important to control for in the random assignment process. For example, if the study includes both male and female participants, stratified random assignment would ensure that there are equal numbers of male and female participants in each group.
To use stratified random assignment, the researcher first identifies the relevant subgroups or strata and then randomly assigns participants within each subgroup to the different groups. This method can help to ensure that the groups are comparable in terms of important characteristics.
Cluster random assignment
Cluster random assignment is useful when participants are not independent of each other (for example, students in a classroom or patients in a hospital). In cluster random assignment, clusters (such as classrooms or hospitals) are randomly assigned to a particular group. All participants within the cluster then receive the same intervention, so the cluster becomes the unit of analysis.
One possible issue with cluster random assignment is that there may be greater within-cluster similarity in outcomes, reducing the power of the study. One approach to counteract this is to increase the number of clusters or to conduct statistical correction to account for the clustering effect.
Conclusion
Random assignment is a critical aspect of experimental design. It ensures that the groups being compared are similar in terms of their characteristics, eliminating any potential pre-existing differences that could bias the results. There are different strategies for random assignment, including simple random assignment, block randomization, stratified random assignment, and cluster random assignment. Choosing the most appropriate method depends on the characteristics of the sample, the research question, and the nature of the intervention being tested. By using appropriate random assignment strategies, psychologists can ensure that their experiments are both rigorous and valid.
References:
Collins LM, Dziak JJ, Li R. (2009). Design of experiments with multiple independent variables: a resource management perspective on complete and reduced factorial designs. Psychol Methods, 14(3): 202-224.
Locke EA, Spirduso WW. (1985). Experimental Design. In: Experimental Research Methods in Human Factors Engineering. Taylor & Francis Group.