The statement of the mean field approximation theorem in the mean field theory of Markov processes particularly targets the behaviour of population processes with an unbounded number of agents. However, in most real-world engineering applications one faces the problem of analysing middle-sized systems in which the number of agents is bounded. In this paper we build on previous work in this area and introduce the mean drift. We present the concept of population processes and the conditions under which the approximation theorems apply, and then show how the mean drift is derived through a systematic application of the propagation of chaos. We then use the mean drift to construct a new set of ordinary differential equations which address the analysis of population processes with an arbitrary size.Population processes are stochastic models of systems which consist of a number of similar agents (or particles) [23]. When the impact of each agent on the behaviour of the system is similar to other agents, it is said that the population process is a mean field interaction model [2]. It is possible to apply a symmetric reduction on the state space of these types of processes and gain some efficiency in their analysis. Mean field approximation refers to the continuous, deterministic approximations of the behaviour of such processes in their first moment, when the number of agents grows very large. These approximations were first proposed for several concrete cases in various areas of study e.g., from as early as the 18th century in population biology, where models such as the predator-prey equations and the SIR equations are used to describe the balance of species in an ecosystem and the dynamics of epidemics respectively [3,13].Since then, general theorems have been proven which show the convergence of the behaviour of population processes to solutions of differential equations. The proofs follow roughly the same steps which generally rely on Grönwall's lemma and martingale inequalities [12]. One of the first generalized approximation theorems was given by Kurtz [21]. The theory gives conditions which define a family of such models called density-dependent population processes, and finds their deterministic approximations by a theorem which is generally called the law of large numbers for standard Poisson processes [15].The mean field theory of Markov processes is increasingly being applied in the fields of computer science and communication engineering. In the field of communication engineering and starting with Bianchi's analysis of the IEEE 802.11 DCF protocol [4,5], much research has focused on discussing the validity of the so-called decoupling assumption in this analysis [7,33,29,11]. Several general frameworks have also been proposed which target the analysis of computer and communication systems [2,25]. In the field of computer science the initial application of the approximations was intuitively motivated by methods such as fluid and diffusion approximations of queueing networks [19]. These have resulted in the de...