The extent of contraceptive adoption by a population is dependent upon a number of factors. As mentioned earlier, in this causal model, the fertility level is the dependent variable, the extent of contraceptive use is the intervening variable, the mortality level is the independent variable, and the unmeasured variables such as attitudes, education, age, religion and the quality of services are all extraneous variables. Without the intervening variable the relationship between the independent and dependent variables will not be complete.