One interesting theoretical extension of the Sraffa model is the examination of the effects of differing applications of labor over time, or the time pattern of production.

We begin by calling the labor currently employed in production direct labor. All of the past labor embodied in the currently employed means of production is called indirect labor (or past labor). The current value of the industry output is, as we already know, equal to the value of the means employed, which is represented by the amount of indirect labor employed times (1 + r) plus the amount of direct labor times the wage w.