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The sample groups used in experimental research are often divided into two groups, the control group and the treatment group. Both these groups are identical in all ways, the only difference being that the treatment group receives the treatment being tested while the control group receives a placebo or nothing at all.
Random assignment is used in experimental research to assign participants to either group using randomization. Therefore, it involves dividing the sample group into the treatment group and the control group using a completely random process. Random assignment is integral to experimental design as it helps construct groups that are comparable.
Random assignment helps build the internal validity of your study as it plays an integral role in controlling different variables, even those that are extraneous.
It helps eliminate any potential sources of bias within the study, and also makes it easier to generalize the results of a study to the larger population. It does so by ensuring members of each group are the same, making them more representative of the larger population, enhancing the external validity of the study in the process.
Random sampling, also known as probability sampling, refers to a broad category of sampling techniques that all involve selecting participants for the sample group using random selection processes.
The concepts of random selection and random assignment are often confused and it is important to understand the difference between the two. Random sampling is used to select participants from your target population to be included in the sample group of your study. Random assignment, on the other hand, involves dividing this sample group into two; the treatment/experimental group and the control group.
Random sampling is often used in many different types of studies, while random assignment is specifically used in between-subjects experimental designs.
There are studies that employ the use of both, some that employ the use of only one, and some that employ the use of neither (cases where non-probability sampling techniques are used in studies that don’t require random assignment).
Random sampling increases the external validity of your research while random assignment increases the internal validity of your study. This is because:
Random assignment can be conducted using very simple techniques. You can start off by assigning a unique number to each member of the sample group. Then, you can use a manual method or an automated method to randomly assign participants to either group. Let’s take a look at some of the different methods used to do so:
This involves placing every number token in a container and then drawing out numbers at random to be assigned to each group.
There are certain computer programs that allow you to generate random numbers. You input all the unique numbers used to label participants, and the program will generate random numbers for you to assign to either group.
If you need to divide your sample into just two groups, you can flip a coin for each number to assign participants into either group.
There are certain cases where random assignment is not applicable or not ethical to employ.
In cases where researchers are studying unhealthy or dangerous behaviours, it is unethical to employ the use of random assignment as it would need to involve the manipulation of unhealthy habits within participants, deteriorating their health in the process. In such cases, quasi-experimental studies can be used. These are studies that do not rely on random assignment, and instead, group participants based on whether they are receiving the treatment or not (without your intervention).
Some studies aim to find differences between participants with certain characteristics. For instance, gender. In such cases, random assignment cannot be used to divide the sample group as participants need to be assigned to groups based on specific characteristics. While studying the difference between men and women, the groups will have to be categorized based on gender.