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

It is important for researchers to distinguish between an independent variable and its conditions. A single independent variable represents the overarching factor being manipulated, while the conditions are the multiple different levels or variations created within that single variable.

Distinguishing Independent Variables from Conditions

A treatment condition is an experimental group in which participants actively receive the specific intervention or treatment being evaluated. To determine whether an intervention is effective, researchers compare the outcomes of participants in this condition against those assigned to a control condition.

Treatment Condition

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

A control condition is an experimental baseline group in which participants do not receive the active treatment or intervention being tested. Researchers compare the outcomes of this untreated group to those in the treatment condition to ascertain if the intervention successfully produced a positive change.

Control Condition

In Darley and Latané’s bystander experiment, the independent variable was the perceived number of witnesses, which researchers manipulated to create three distinct conditions: participants believed there were either one, two, or five other students present. To maintain experimental control, the researchers held extraneous variables constant by testing everyone in the exact same room and exposing them to the identical simulated emergency. They also randomly assigned participants to these conditions to ensure the groups were similar from the start, effectively controlling for extraneous participant variables.

Example of Conditions and Control in Darley and Latané's Experiment

A single factor two-level design is an experiment that involves a single independent variable manipulated to create exactly two distinct conditions. For example, a researcher might investigate the effects of expressive writing by manipulating the independent variable to form two groups: a traumatic experience condition and a neutral experience condition.

Single Factor Two-Level Design

In an experiment, researchers systematically create distinct levels or variations of a specific factor to study its effects. What is the term used for these distinct levels?

In an experiment, conditions refer to the distinct levels or variations of the independent variable that the researcher systematically creates through manipulation. Researchers typically assign these conditions short, descriptive names to make them easier to discuss. For example, a study investigating the effects of expressive writing on health might create a 'traumatic condition' and a 'neutral condition'.

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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

In experimental design, an independent variable is the presumed cause within a cause-and-effect relationship. It is the specific factor that researchers actively manipulate or systematically vary to observe what change it produces in another 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

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An experiment can investigate how technology use in the classroom affects learning by comparing note-taking methods. In this scenario, the independent variable is the type of note-taking by participants, such as using a laptop versus writing by hand. The dependent variable is the learning exhibited by participants, measured by their performance on an assessment to evaluate the impact of the independent variable.

Example of an Experiment: Note-Taking Method and Learning Performance

In experimental research, the dependent variable is expected to change as a direct function of the independent variable. This means the value or state of the dependent variable is contingent upon the manipulation of the independent variable. A helpful way to conceptualize this relationship is by asking the question: 'What effect does the independent variable have on the dependent variable?'

Functional Relationship Between Independent and Dependent Variables

A researcher conducts a study to determine if the amount of sleep affects problem-solving skills. One group of participants is allowed to sleep for eight hours, while a second group is only allowed to sleep for four hours. The next morning, both groups are given the same set of puzzles to solve, and the researcher records how many puzzles each participant completes correctly. In this study, what is the independent variable?

Which pair of terms correctly fills in the blanks in the following statement?

The ________ is controlled by the experimenter, while the ________ represents the information collected and statistically analyzed by the experimenter.

- dependent variable; independent variable
- independent variable; dependent variable
- placebo effect; experimenter bias
- experiment bias; placebo effect

Identifying Experimental Variables

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

In psychological research, a treatment is defined as any intervention intended to change people's behavior for the better. This broad concept encompasses medical treatments and psychotherapies for psychological disorders, as well as various interventions designed to reduce prejudice, promote conservation, or improve learning.

Treatment

Non-experimental research is a broad category of research methodologies defined by the lack of manipulation of an independent variable. Instead of controlling factors, researchers simply measure variables as they naturally occur in real-world or laboratory settings. Although experimental research can provide strong evidence that changes in an independent variable cause differences in a dependent variable, non-experimental research generally cannot. Despite this inability to draw causal conclusions, non-experimental research is still considered extremely important and is utilized in cases where experimental research cannot be carried out.

Non-Experimental Research

A non-manipulated independent variable is a factor in an empirical study that the researcher measures but does not actively manipulate or alter. These are frequently participant variables—such as gender, self-esteem, or hypochondriasis—which naturally function as between-subjects factors because an individual can only be tested in one condition of their pre-existing trait. A study incorporating these factors is still classified as an experiment provided that at least one other independent variable is actively manipulated.

Non-Manipulated Independent Variable

An operational definition defines a psychological variable in terms of the exact procedures used to measure or observe it. Because most variables are abstract constructs that cannot be directly observed (e.g., stress), researchers must transform these conceptual definitions into concrete, quantifiable forms. For instance, stress could be operationally defined as a score on a standardized questionnaire, the concentration of cortisol in saliva, or the number of recent stressful life events experienced.

Operational Definition

Experimental Condition

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 experimental research, an independent variable often represents an abstract construct that cannot be manipulated directly. To address this, researchers utilize a manipulation check, which is a separate, secondary measurement of the targeted construct. The primary purpose of this procedural step is to verify whether the independent variable was successfully altered in the way the researcher intended.

Manipulation Check

In experimental research, a null result occurs when the collected data reveal no statistically significant effect of the manipulation of the independent variable on the dependent variable.

Null Result

In an experimental research study, how is an independent variable defined?

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

Just as psychological studies frequently utilize multiple levels of a single independent variable, it is also a common and powerful practice to incorporate multiple independent variables within the exact same experiment. This methodological approach enables researchers to answer more sophisticated research questions that cannot be addressed by studying a single variable in isolation.

Multiple Independent Variables

In an experimental design, which of the following best describes an independent variable?

A true experiment requires the researcher to actively intervene and manipulate the independent variable; merely comparing pre-existing groups does not constitute manipulation. For instance, comparing the health of people who already keep a journal with those who do not is not an experiment, because these groups likely differ in other confounding ways (such as conscientiousness or stress levels). Active manipulation is crucial for eliminating these potential alternative explanations for the results.

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