Intervening Variables in Psychology In the real world, variables often don't stand alone. In many cases, factors affect the main variables of interest.

Elliott Sober, Black Box Inference: When Should Intervening Variables Be Postulated?, The British Journal for the Philosophy of Science, Vol. 49, No. 3 (Sep., 1998), pp. 469-498 ...

Mario Renzi, Ideal Family Size as an Intervening Variable between Religion and Attitudes Towards Abortion, Journal for the Scientific Study of Religion, Vol. 14, No. 1 (Mar., 1975), pp. 23-27 ...

Intervention variables can have any pattern--impulse and continuing interventions are just two possible cases. You can mix discrete valued intervention variables and continuous regressor variables in ...