## 1 Introduction  
Designing efficient assembly processes requires computing the number of people necessary to accomplish the task and then assigning work elements to operators in an even manner. This is especially important in a lean management (LM) context, which requires products to be completed in a just-in-time fashion, without _muda_ (Japanese for wasted resources in non-value-adding work), _muri_ (people overburden) or _mura_ (workload variation) [1]. Thus, this paper develops a procedure that balances the scheduled workload with the capacity of a process in order to compute the minimum number of operators to finish the job in the allowed time (e.g., a workday) and then it determines the time necessary to complete each model when more than one model is made during the same workday. \"Lean\" was the word used by researchers at the Massachusetts Institute of Technology (MIT), in the 1980s, to describe the efficient plants of Japanese car manufacturers [2]. A fruit of their research was the book \"The machine that changed the world\" [3], which greatly contributed to the diffusion of LM among practitioners. LM has its roots in the Toyota Production System (TPS) [3], which was developed on the basis of two principles: respect for people (or employee engagement) and just-in-time (JIT) production [4]. For this reason, TPS, JIT and LM are frequently used as synonyms in the literature. For the purpose of this paper, LM can be defined as a management approach that tries to identify and remove all types of _muda_, _muri_ and _mura_ from the business processes in order to increase efficiency, cut costs and reduce order lead time. LM includes a variety of industrial practices such as single-piece flow (or, at least, small lot size), leveling of production (every day a stable mix of different models is made in order to meet customer demand) and setup time reduction techniques, which allow operators to quickly move from assembling one model to another [4; 5; 6; 7]. An assembly line is a manufacturing process in which parts are added (or operations are performed) to a succession of products as these products visit workstations successively according to a previously defined sequence. Usually, workstations are aligned and placed close to one another. Products may be moved by a conveyor belt or another transportation system between stations. In an assembly line, workers (or even robots) are assigned specific operations, which they repeat before the product moves to the next station in the line. The assembly line devised by [PERSON] in the early days of the twentieth century was intended for one standardized product--the Ford Model T-- but, currently, several models may be assembled on the same line. Consequently, assembly lines are classified either as \"single-model\", \"multi-model\" or \"mixed-model\" lines [8]. The single-model line is dedicated to the production of one single model of an end product (e.g., one type of car engine). In multi-model lines, different models of an end product are produced separately in batches. For example, in a day, first, 20 units of model A are assembled, one by one; later, as a batch, 30 units of model B are assembled on the same line, and after that, 15 units of model C are assembled, one after the other, on the same line too. The operation of a multi-model line frequently includes intermediate setup operations (e.g., time to change fixtures) between different models. For this reason, it is convenient to make the different models in batches in order to have small setup times compared to the lot processing time. In mixed-model lines, different models can be produced in any order [9] and the batch size can be as small as one. For example, one unit of A; after, one unit of B; and then two units of C. The sequence may be adjusted to market demand and/or it may try to smooth the demand for upstream components. Obviously, this manufacturing strategy can only be applied if setup times in between are not significant; otherwise, more time would be spent in unproductive setup than in assembly work.