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The Definition of Random Assignment According to Psychology

Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

Emily is a board-certified science editor who has worked with top digital publishing brands like Voices for Biodiversity, Study.com, GoodTherapy, Vox, and Verywell.

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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 in a study to eliminate any potential bias in the experiment at the outset. Participants are randomly assigned to different groups, such as the treatment group versus the control group. In clinical research, randomized clinical trials are known as the gold standard for meaningful results.

Simple random assignment techniques might involve tactics such as flipping a coin, drawing names out of a hat, rolling dice, or assigning random numbers to a list of participants. It is important to note that random assignment differs from random selection .

While random selection refers to how participants are randomly chosen from a target population as representatives of that population, random assignment refers to how those chosen participants are then assigned to experimental groups.

Random Assignment In Research

To determine if changes in one variable will cause changes in another variable, psychologists must perform an experiment. Random assignment is a critical part of the experimental design that helps ensure the reliability of the study outcomes.

Researchers often begin by forming a testable hypothesis predicting that one variable of interest will have some predictable impact on another variable.

The variable that the experimenters will manipulate in the experiment is known as the independent variable , while the variable that they will then measure for different outcomes is known as the dependent variable. While there are different ways to look at relationships between variables, an experiment is the best way to get a clear idea if there is a cause-and-effect relationship between two or more variables.

Once researchers have formulated a hypothesis, conducted background research, and chosen an experimental design, it is time to find participants for their experiment. How exactly do researchers decide who will be part of an experiment? As mentioned previously, this is often accomplished through something known as random selection.

Random Selection

In order to generalize the results of an experiment to a larger group, it is important to choose a sample that is representative of the qualities found in that population. For example, if the total population is 60% female and 40% male, then the sample should reflect those same percentages.

Choosing a representative sample is often accomplished by randomly picking people from the population to be participants in a study. Random selection means that everyone in the group stands an equal chance of being chosen to minimize any bias. Once a pool of participants has been selected, it is time to assign them to groups.

By randomly assigning the participants into groups, the experimenters can be fairly sure that each group will have the same characteristics before the independent variable is applied.

Participants might be randomly assigned to the control group , which does not receive the treatment in question. The control group may receive a placebo or receive the standard treatment. Participants may also be randomly assigned to the experimental group , which receives the treatment of interest. In larger studies, there can be multiple treatment groups for comparison.

There are simple methods of random assignment, like rolling the die. However, there are more complex techniques that involve random number generators to remove any human error.

There can also be random assignment to groups with pre-established rules or parameters. For example, if you want to have an equal number of men and women in each of your study groups, you might separate your sample into two groups (by sex) before randomly assigning each of those groups into the treatment group and control group.

Random assignment is essential because it increases the likelihood that the groups are the same at the outset. With all characteristics being equal between groups, other than the application of the independent variable, any differences found between group outcomes can be more confidently attributed to the effect of the intervention.

Example of Random Assignment

Imagine that a researcher is interested in learning whether or not drinking caffeinated beverages prior to an exam will improve test performance. After randomly selecting a pool of participants, each person is randomly assigned to either the control group or the experimental group.

The participants in the control group consume a placebo drink prior to the exam that does not contain any caffeine. Those in the experimental group, on the other hand, consume a caffeinated beverage before taking the test.

Participants in both groups then take the test, and the researcher compares the results to determine if the caffeinated beverage had any impact on test performance.

A Word From Verywell

Random assignment plays an important role in the psychology research process. Not only does this process help eliminate possible sources of bias, but it also makes it easier to generalize the results of a tested sample of participants to a larger population.

Random assignment helps ensure that members of each group in the experiment are the same, which means that the groups are also likely more representative of what is present in the larger population of interest. Through the use of this technique, psychology researchers are able to study complex phenomena and contribute to our understanding of the human mind and behavior.

Lin Y, Zhu M, Su Z. The pursuit of balance: An overview of covariate-adaptive randomization techniques in clinical trials . Contemp Clin Trials. 2015;45(Pt A):21-25. doi:10.1016/j.cct.2015.07.011

Sullivan L. Random assignment versus random selection . In: The SAGE Glossary of the Social and Behavioral Sciences. SAGE Publications, Inc.; 2009. doi:10.4135/9781412972024.n2108

Alferes VR. Methods of Randomization in Experimental Design . SAGE Publications, Inc.; 2012. doi:10.4135/9781452270012

Nestor PG, Schutt RK. Research Methods in Psychology: Investigating Human Behavior. (2nd Ed.). SAGE Publications, Inc.; 2015.

By Kendra Cherry, MSEd Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

AP Psychology Community

Random assignment

Assigning participants to experimental and control conditions by chance, thus minimizing preexisting differences between those assigned to the different groups.

Random sample

A sample that fairly represents a population because each member has an equal chance of inclusion

The difference between the highest and lowest scores in a distribution.

Rationalization

Defense mechanism that offers self-justifying explanations in place of the real, more threatening, unconscious reasons for one’s actions.

Reaction formation

Defense mechanism by which the ego unconsciously switches unacceptable impulses into their opposites. Thus, people may express feelings that are the opposite of their anxiety-arousing unconscious feelings.

A measure of memory in which the person must retrieve information learned earlier, as on a fill-in-the-blank test.

Reciprocal determinism

The interacting influences between personality and environmental factors.

Recognitions

A measure of memory in which the person need only identify items preciously learned, as on a multiple-choice test.

A simple, automatic, inborn response to a sensory stimulus, such as the knee-jerk response.

Refractory period

A resting period after orgasm, during which a man cannot achieve another orgasm.

Defense mechanism in which an individual faces with anxiety retreated to a more infantile psychosexual stage where some psychic energy remains fixated.

Regression toward the mean

The tendency for extremes of unusual scores to fall back (regress) toward the average.

The conscious repetition of information, either to maintain it in consciousness or to encode it for storage.

In operant condition, any even that strengthens the behavior it follows.

Relative deprivation

The perception that one is worse off relative to those with whom one compares oneself.

A memory measure that asses the amount of time saved when learning material for a second time.

Reliability

The extent to which a test yields consistent results, as assessed by the consistency of scores on two halves of the test, on alternate forms of the test, or on retesting.

REM rebound

The tendency for REM sleep to increase following REM sleep deprivation (created by repeated awakening during REM sleep).

Rapid eye movement sleep, a recurring sleep stage during which vivid dreams commonly occur. Also, known as paradoxical sleep because the muscles are relaxed (Accept for minor twitched) but other body systems are active.

Replication

Repeating the essence of a research study, usually with different subjects in different situations to see whether the basic finding generalized to other participants and circumstances.

Representativeness heuristic

A rule of thumb for judging the likelihood of things in terms of how well they seem to represent, or match, particular prototypes; may lead one to ignore other relevant information.

In psychoanalytic theory, the basic defense mechanism that banishes anxiety-arousing thoughts, feelings, and memories from consciousness.

In psychoanalysis, the blocking from consciousness of anxiety-laden material.

Respondent behavior

Behavior that occurs as an automatic response to some stimulus; Skinner’s term for behavior learned through classical conditioning.

Reticular formation

A nerve network in the brainstem that plays an important role in controlling arousal.

The light-sensitive inner surface of the eye, containing the receptor rods, and cones, plus layers of neurons that begin the process of visual information.

Retinal disparity

A binocular cue for perceiving depth: The greater the disparity (difference) between the two images, the retina receives of an object, the closer the object is to the viewer.

The process of getting information out of memory storage.

Retroactive interference

The disruptive effect of new learning of the recall of old information.

Retinal receptors that detect black, white, and grey; necessary for peripheral and twilight vision, when cones do not respond.

A set of expectation (norms) about a social position, defining how those in the position ought to behave.

Rooting reflex

A baby’s tendency, when touched on the cheek, to open the mouth, and search for the nipple.

Rorschach inkblot test

The most widely used projective test, a set of 10 inkblots, designed by Hermann Rorschach; seeks to identify people’s inner feelings, by analyzing their interpretation of the blots.

All Subjects

AP Statistics

Study guides for every class, that actually explain what's on your next test, random assignment, from class:.

Random assignment is a technique used in experiments to randomly allocate participants into different groups, ensuring that each participant has an equal chance of being placed in any group. This process helps eliminate bias and ensures that any differences observed between the groups can be attributed to the treatment rather than pre-existing differences. By using random assignment, researchers can make causal inferences about the effects of the treatment being tested.

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5 Must Know Facts For Your Next Test

Random assignment is crucial for ensuring internal validity in experimental research, as it minimizes the influence of confounding variables.
This technique allows for the use of statistical methods to analyze data and determine if observed effects are significant.
Random assignment differs from random selection; while random selection refers to how participants are chosen for a study, random assignment deals with how they are placed into groups after selection.
In practice, random assignment can be achieved using methods like random number generators or drawing lots.
By creating equivalent groups through random assignment, researchers can enhance the credibility of their findings and support causal claims.

Review Questions

Random assignment enhances the validity of an experiment by ensuring that participants are equally likely to be assigned to any group. This equality helps eliminate biases and confounding variables that could affect the outcome. As a result, researchers can more confidently attribute differences in results directly to the treatment or intervention applied, rather than pre-existing characteristics among participants.
Random assignment and random selection serve different purposes in research. Random selection involves choosing participants from a larger population to ensure that the sample is representative, which is important for generalizability. In contrast, random assignment allocates these selected participants into different groups within an experiment, enhancing internal validity by controlling for confounding variables. Both processes are essential but focus on different stages of study design.
Not using random assignment can severely compromise the integrity of an experimental study. Without it, groups may differ significantly due to confounding variables or biases, making it difficult to establish a clear cause-and-effect relationship. As a result, any conclusions drawn may be misleading or invalid. Researchers risk attributing effects of a treatment to factors unrelated to the intervention, which undermines the reliability and credibility of their findings.