Measured Variables The standard independent variable influences the outcome and is measured by the researcher. A measured variable is an independent variable that is measured or observed by the researcher and consists of a range of continuous or categorical scores. For example, consider the following research question:How does math ability influence achievement on the final quiz in the classroom?The independent variable is a measured variable indicating math ability scores assessed by results on an ability test.Control Variables A control variable is another type of independent variable that researchers measure for the purposes of eliminating it as a possibility, but it is not a central variable of concern in explaining the dependent variables or outcomes. A control variable is a variable that is important to consider and “neutralize” (Tuckman, 1999, p. 100) because it potentially influences the dependent variable. Typically, control variables are personal demographic attributes or characteristics (Tuckman, 1999) such as:◆ Gender ◆ Intelligence ◆ Socioeconomic status ◆ RaceThese variables are typically controlled through statistical procedures. Later we will learn that these variables are called covariates and they are statistically adjusted for their effects.Treatment VariablesTwo other specific types of independent variables can be introduced here because they will be used in educational experiments. In an experiment, a researcher treats one group of participants to specific activities and withholds them from another group. The question is whether the group that receives the activities scores differently on the dependent variable than the group without the activities. Because researchers assign individuals to these two groups, the groups are “treated” or “manipulated” by the researcher. A treatment variable is measured in categories (received or denied activities) to determine its effect on an outcome. In the literature you will find these variables labeled as manipulated variables or variables with levels. Experimental researchers refer to these groups as levels (i.e., Group 1, Group 2).In the following example, the treatment variable is the type of instruction used by the teacher in an elementary math classroom:In a study of student achievement outcomes in an elementary math classroom, the researcher gives one group small group discussion (Level 1) and another traditional group lecture (Level 2) to assess the independent variable, type of instruction.Independent treatment variable: Type of instruction is considered a treatment variable because the researcher intervenes with one group, Level 1.Moderating Variables Moderating variables deserve our attention because they, too, are often used in educational experiments. Moderating variables are new variables costructed by the researcher by taking one variable times another to determine the joint impact of both variables together. This impact is called an interaction effect. For now, recognize that interaction effects are a special form of independent variable. A moderating variable can be illustrated in this quantitative hypothesis:Small-group discussion for students with high prior test scores contributes to higher math quiz results than lecture discussion for students with low prior test scores.
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