A placebo is a simulated treatment that lacks any active ingredient or specific element intended to make it effective. Despite lacking active components, placebos are designed to mimic the appearance or experience of actual treatments, such as an identical-looking sugar pill in a drug trial or an unstructured conversation with a therapist in psychotherapy research.

Placebo

Random assignment is the primary method used to control extraneous variables across experimental conditions. It involves using a chance-based procedure to determine which participants are placed into which conditions. Strictly speaking, it requires that every participant has an equal probability of being placed in any condition and that each individual's assignment is completely independent of the others.

Random Assignment

What is the primary purpose of experimental control in a research study?

Experimental control is the fundamental process by which researchers hold extraneous variables constant throughout a study. Because extraneous factors—such as individual participant differences or situational contexts—are likely to have an unintended effect on the dependent variable, they make it difficult to separate their impact from the true effect of the manipulated independent variable. By actively controlling these outside variables and preventing them from fluctuating across conditions, researchers can isolate the independent variable and confidently attribute any changes in the dependent variable to the experimental manipulation.

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In experimental research, an extraneous variable is any factor that varies within the context of a study other than the specific independent and dependent variables being investigated. These variables pose a significant problem because many of them are likely to influence the dependent variable. If these outside factors fluctuate, they introduce competing influences that make it difficult to separate the true effect of the independent variable from the effects of the extraneous variables.

Extraneous Variable

An experiment is a specific type of research study designed to determine if a meaningful relationship between two variables is causal in nature. By utilizing statistical analysis to support these causal conclusions, experiments serve as one of the most common and valuable tools in psychological research.

Experiment

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A participant variable, also known as an individual difference, is a type of extraneous variable derived from the pre-existing characteristics of the individuals involved in an experiment. Examples of these variables include demographic factors such as gender, as well as personal attributes or habits like diet and writing ability. Because these individual differences are likely to have their own independent effect on the dependent variable, they must be controlled to prevent them from obscuring the effect of the experimental manipulation.

Participant Variable

A situational variable is a type of extraneous variable that involves the broader environmental or contextual conditions present during an experiment, such as the time of day, weather, or physical setting. Since these external factors can unintentionally influence participants' behavior and the dependent variable, researchers must rigorously control them to isolate the true effect of the independent variable.

Situational Variable

A task variable is a specific type of extraneous variable relating to the mechanics, materials, or procedures of the activity participants are asked to complete during a study. For example, whether a participant completes a task by hand or on a computer is a task variable. Because variations in how a task is administered can independently affect the dependent variable, they must be held constant.

Task Variable

Experimental Control

The biological sex of a researcher can act as an extraneous variable by unintentionally influencing participant behavior. Because male and female experimenters often interact differently with subjects—and subjects respond differently to them—this variation can confound study results. For instance, in a pain perception study by Ibolya, Brake, and Voss (2004), participants immersed their hands in icy water longer when the experimenter was of the opposite sex, demonstrating that researcher characteristics must be controlled to prevent noise in the data.

Experimenter's Sex as an Extraneous Variable

Standardizing the procedure is the process of minimizing unintended variation by conducting an experiment in the exact same way for all participants, regardless of their assigned condition. This consistency prevents extraneous variables, such as experimenter demeanor, from systematically affecting the results and creating confounds. Common standardization techniques include using written protocols, delivering standard or automated instructions, anticipating participant questions in advance, training multiple experimenters together, and ensuring each experimenter tests participants in all conditions.

Standardizing the Procedure

The experimenter expectancy effect occurs when a researcher's own expectations or beliefs about a study's outcome inadvertently bias the participants' behaviors in unintended ways. These subtle influences during the administration of a measure can systematically alter participant responses and compromise the study's validity.

Experimenter Expectancy Effect

In an experiment testing the effect of expressive writing on physical health, researchers must manage multiple extraneous variables. Participant variables (individual differences) like baseline writing ability, gender, and diet can independently affect health. Furthermore, situational and task variables, such as the weather, the time of day, and whether participants type or write by hand, can introduce unintended variability. Holding these factors constant is necessary to separate the actual impact of expressive writing from the influence of these outside variables.

Example of Extraneous Variables: Expressive Writing Experiment

A confounding variable is an extraneous variable that differs on average across levels of the independent variable, meaning it varies systematically alongside it. Because this variable differs systematically across conditions just like the independent variable, it provides a plausible alternative explanation for any observed difference in the dependent variable, making it difficult to establish a clear cause-and-effect relationship. For instance, if an extraneous variable like intelligence is substantially lower in one group than another, it becomes a confounder that confuses the results.

Confounding Variable

Internal validity indicates the degree to which an empirical study's design justifies the conclusion that changes in the independent variable directly caused the observed differences in the dependent variable. Experimental research designs typically achieve high internal validity because they involve deliberately manipulating an independent variable while rigorously controlling for extraneous variables, thereby eliminating alternative causal explanations. For instance, in the Darley and Latané experiment, the rigorous manipulation of the number of perceived bystanders isolated this independent variable as the direct cause of differences in helping behavior. In contrast, non-experimental correlational designs exhibit low internal validity; for example, a statistical relationship between regular exercise and happiness cannot confirm that exercise causes happiness, as other factors or reverse causation might explain the link.

Internal Validity

In the context of an experiment, what is an extraneous variable?

In experimental research, an independent variable is the presumed cause in a cause-and-effect relationship. It is the specific factor that a researcher systematically manipulates or varies to observe its impact on another variable. While experiments require manipulation, in some non-experimental or factorial designs, an independent variable may simply be measured.

Independent Variable

A dependent variable is the factor in an experiment that represents the presumed effect within a causal relationship. It is the variable that researchers measure to evaluate if it changes as a direct result of the manipulation of the independent variable.

Dependent Variable

To manipulate an independent variable means to systematically change its level so that different groups of participants are exposed to different levels of that variable, or the same group of participants is exposed to different levels at different times. This active manipulation is a fundamental requirement of an experiment and is used to create distinct conditions for comparison.

Experimental Manipulation

A within-subjects experiment is an experimental design strategy where every participant is exposed to all levels of the independent variable. This approach contrasts with between-subjects designs by testing the exact same individuals across every experimental condition instead of assigning them to just one.

Within-Subjects Experiment

A between-subjects experiment is an experimental design approach wherein each participant is exposed to only a single level of the independent variable. This method differs fundamentally from within-subjects designs, as individuals are tested in just one specific condition rather than across all experimental variations.

Between-Subjects Experiment

A pilot test is a small-scale preliminary study conducted prior to a full experiment to ensure that a new research procedure functions exactly as planned. By running this trial, researchers can identify logistical issues, clarify instructions, and refine their methodology before committing resources to the main investigation.

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