The intervening variables are abstract processes not directly observable, but which link the independent and dependent variables. Language learning and teaching typically take place inside the subjects’ heads, including various processes of language acquisition that can’t be observed by the researcher. It is a hypothetical variable used to explain causal relationships between variables. Experimental variables cannot be observed (that’s why they are hypothetical).
The intervening variable is sometimes referred to as a mediating variable or intermediary variable. An intervening variable is usually considered a subtype of a mediating variable in statistics. These terms have been used interchangeably for a long time, however, and the lines between them are somewhat blurry. In research terminology the intervening variable can be described as