Independent Variable

Any research endeavor, accompanied by the use of statistical and measurement tools, often finds one of several different types of variables present, referred to as independent variables.

An independent variable is the variable that is manipulated or changed to gauge the effects on some outcome or dependent variable—it is the “force” that is hypothesized to cause change in the outcome of the experiment. It is also sometimes called a treatment variable because it is often the treatment or the experimental condition that is applied at different levels (hence, it is a variable) to a selected group, and then the effects of that application are evaluated through an examination of the outcome or the effects on the dependent variable.

For example, one can think of a set of independent variables (in this example, three) acting alone and together (in their interaction) to influence some outcome (usually called an independent variable) as follows:

where

• DV is the value of the dependent variables
• f is the function of sign
• IV1 is the first independent variable, with other independent variables (IVs) such as IV2 and IV3 to follow.

Table 1 is a simple experimental design where the one independent variable is number of hours of reading per week that a group of children receives in an after-school program. The dependent variable is the children's comprehension, which is evaluated using a test that assesses that variable.

Each of the three groups (one for each level of the independent variable) receives one of the three treatments, and each level of treatment represents one of the levels of the independent variable, which is labeled Amount of Extra Reading.

Table 1 An Experimental—Control Group Comparison
		Amount of Extra Reading
	No Extra Reading	5 Hours Extra Reading	10 Hours Extra Reading
Control Group
Experimental Group

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What are independent variables independent of? One another—the most efficient independent variables are those that act on their own and contribute a better understanding of the dependent variable. Ideally, each independent variable is unrelated to the others, so that its contributions are unique. If the variables are related to one another, it is difficult to separate the effects of each one on the dependent variable and, therefore, to clearly conclude that the change to the dependent variable is a function of any one independent variable rather than a combination.

Applying Ideas on Statistics and Measurement

The following abstract is adapted from Mohr, W. K., Lutz, M. N., Fantuzzo, J. W., & Perry, M. A. (2000). Children exposed to family violence: A review of empirical research from a developmental-ecological perspective. Trauma, Violence, & Abuse, 1(3), 264–283.

There are many different types of variables, independent or treatment variables being one of them. In this review, Wanda Mohr and her colleagues focused on a developmental-ecological model to evaluate the past decade of research on children exposed to family violence, and a coding system was applied to all empirical studies published in this area since 1987. This coding system allowed for inspection of the current state of the knowledge base from this perspective and the ability to determine the progress that has been made in this research area. A total of 21 quantitative studies and one qualitative study were reviewed. Despite progress over the past 10 years, foundational issues related to definitions of the independent variable, substantiation of exposure, developmental sensitivity, and methodology remain. The authors also present recommendations for future research studies.

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