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Variables
A variable may be defined as a concept or a characteristic that involves variations. Researchers use variable rather than concept or characteristic because most of what is studied varies, that is, involves variation that can be described numerically or categorically.
Variables are composed of attributes or levels. An attribute is the value or category that makes up the variation. Thus, for example, the variable gender would have as attributes female and male. These categories may also be referred to as levels.
A precise definition of each variable communicates clearly the researcher's intent and enhances the usefulness of the results. Two types of definition of variables are commonly used in research: conceptual and operational.
A conceptual definition uses other words and concepts to describe the variable, as in a dictionary. Conceptual definition is important in communicating what is being investigated but may not indicate precisely what the variable means.
An operational definition indicates how the concept is measured or manipulated, that is, what operations are performed to measure or manipulate the variable.
There are several types of variables in educational research. The most important are independent and dependent, extraneous and confounding, and continuous and categorical.
In research, one variable precedes another, either logically or in time. The variable that comes first and influences or predicts is called the independent variable. The second variable, the one that is affected or predicted by the independent variable, is the dependent variable. In an experiment, at least one independent variable is the presumed cause of differences between groups on the dependent variable. The independent variable is the antecedent, and the dependent variable is the consequence. In nonexperimental research, the independent variable cannot be manipulated or controlled by the investigator. Such variables may still be considered independent if they clearly precede the dependent variable or if they are used to create categories for comparison.
Extraneous and Confounding Variables
An extraneous variable affects the dependent variable but is unknown or not controlled by the researcher. These variables change the result and disrupt the study. In designing research, investigators try to control or account for whatever extraneous variables may be present.
A confounding variable is one that varies systematically with the independent variable.
Continuous and Categorical Variables
A continuous variable can theoretically take on an infinite number of values within a given range of scores. In other words, the value of a continuous variable could be any point on a continuum. The values are rank ordered, from small to large or low to high, to indicate some properties or characteristics. Common continuous variables in education research are achievement and aptitude test scores.
A categorical variable is used to assign an object or person to a group that is defined to have specific characteristics. The simplest type of category has two groups (dichotomous), such as male/female. Other categorical variables can have three or more groups, for example, grade level and nationality. It is also common to use continuous variables to create categories. For instance, an aptitude test score is generally treated as a continuous variable, but the scores can be grouped into categories such as high, middle, and low categories. Thus, depending on how the researcher uses the variable, it can be either continuous or categorical.
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