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The things that are changing in an experiment are called variables. A variable is any factor, trait, or condition that can exist in differing amounts or types.
DEPENDENT VARIABLES show the effect of manipulating or introducing the independent variables. For example, if the independent variable is the use or non-use of a new language teaching procedure, then the dependent variable might be students' scores on a test of the content taught using that procedure. In other words, the variation in the dependent variable depends on the variation in the independent variable.
INDEPENDENT VARIABLES are those that the researcher has control over. This "control" may involve manipulating existing variables (e.g., modifying existing methods of instruction) or introducing new variables (e.g., adopting a totally new method for some sections of a class) in the research setting.
Whatever the case may be, the researcher expects that the independent variable(s) will have some effect on (or relationship with) the dependent variables.
INTERVENING VARIABLES refer to abstract processes that are not directly observable but that link the independent and dependent variables. In language learning and teaching, they are usually inside the subjects' heads, including various language learning processes which the researcher cannot observe. For example, if the use of a particular teaching technique is the independent variable and mastery of the objectives is the dependent variable, then the language learning processes used by the subjects are the intervening variables.