1.3 Types of variables and their roles in experiments

Variables are the building blocks of experiments. They come in different types, each playing a unique role. Independent variables are manipulated by researchers, while dependent variables are measured in response. Understanding these distinctions is crucial for designing effective experiments.

Control variables are held constant to isolate the effect of independent variables. Confounding variables can muddy results if not accounted for. Moderating and mediating variables help explain the nuances of relationships between variables. Recognizing these roles enhances experimental design and interpretation.

Types of Variables

Independent and Dependent Variables

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• Independent variable is the variable that is manipulated or changed by the researcher
• Independent variable is the presumed cause in an experiment
• Dependent variable is the variable that is measured or observed in response to the independent variable
• Dependent variable is the presumed effect in an experiment
• In a study on the effect of fertilizer on plant growth, the amount of fertilizer would be the independent variable (manipulated by researcher) and plant height would be the dependent variable (measured in response)

Categorical and Continuous Variables

• Categorical variables are variables that can be divided into distinct categories or groups
• Categorical variables can be nominal (categories with no inherent order, such as eye color) or ordinal (categories with a natural order, such as rankings)
• Continuous variables are variables that can take on any value within a certain range
• Continuous variables are often measured on a scale, such as height, weight, or temperature
• In a survey about favorite ice cream flavors, flavor would be a categorical variable (distinct categories like vanilla, chocolate, strawberry) while rating of enjoyment on a scale from 1-10 would be a continuous variable

Control and Confounding Variables

Control Variables

• Control variables are variables that are held constant throughout an experiment to minimize their effect on the dependent variable
• Researchers control these variables to isolate the effect of the independent variable on the dependent variable
• Control variables help to ensure that any changes in the dependent variable are due to the manipulation of the independent variable and not some other factor
• In an experiment on the effect of light on plant growth, temperature and soil type would be control variables (kept the same for all plants) to isolate the effect of light

Confounding and Extraneous Variables

• Confounding variables are variables that are related to both the independent and dependent variables, making it difficult to determine the true effect of the independent variable
• Confounding variables can lead to misleading conclusions if not accounted for in the experimental design
• Extraneous variables are any other variables that may affect the dependent variable but are not of primary interest in the study
• Researchers try to minimize the influence of extraneous variables through control and randomization
• In a study on the effect of a new drug on blood pressure, age could be a confounding variable (older people tend to have higher blood pressure and may respond differently to the drug) while noise level in the testing environment would be an extraneous variable

Interacting Variables

Moderating Variables

• Moderating variables are variables that affect the strength or direction of the relationship between the independent and dependent variables
• Moderating variables can enhance, reduce, or change the effect of the independent variable on the dependent variable
• Researchers often study moderating variables to understand when or for whom an effect is strongest
• In a study on the effect of stress on job performance, coping skills could be a moderating variable (people with better coping skills may perform better under stress compared to those with poor coping skills)

Mediating Variables

• Mediating variables are variables that explain how or why an independent variable affects a dependent variable
• Mediating variables are the mechanism through which the independent variable influences the dependent variable
• Mediating variables are often studied to understand the underlying process or pathway of an effect
• In a study on the effect of education on income, job skills could be a mediating variable (education leads to better job skills, which in turn leads to higher income)