Control or Eliminate Extraneous Variance:

It means the control or elimination of the impact of other independent variables which are likely to influence the dependent variable but are considered as unwanted or extraneous in teh context of particular research work. It is essential to eliminate their impact so that variation in the dependent variable takes place only because of the assumed case. In case of the above example age intelligence previous school environment family background etc are the additional factor which might influence the reading habit or the class performance of the students.

The researcher job is to isolate minimize nullify or eliminate the effect of such extraneous variable in order to demonstrate the effect of experimental variable teaching methods on the dependent variable the students class performance of the students reading habit.