this therefore means that it is important to first clarify the different perspectives represented herein for example creation of multilevel framework including the analysis of well-established literature that is related to this discussion . Womack, Jones and Roos (1990) in their canonical work The Machine that Changed the World were the first ones to formally introduce Lean Product Development. Although they mainly focused on manufacturing and assembly processes, they still describe some tools and methods that are used to achieve Lean performance in Product Development including Concurrent Engineering. Consequently it should be noted that there is a strong connection between Lean Product Development and Concurrent Engineering.

Concurrent Engineering has been defined by Winners et al. (1998) as the systematic approach that integrates the concurrent designs of the different products in relation to their manufacturing and supporting processes with the intention to cause developers of these products to consider all the elements that are put together to come up with the products from conception to disposal of the same. These include quality, cost of production as well as schedule and requirements of the user.

According to Maylor (1997), although Concurrent engineering has received a lot of attention from both researchers and practitioners, its application in the industry seems to be minimal. Moreover the surveys conducted by Ainscough and Yazdani (1999) show that although three quarters of aerospace companies and about two thirds of industries claim to use concurrent engineering to some extent, only just below one third of those interviewed said that they may have the ability to fully implement concurrent engineering across the entire company. This according to Haque and James -- Moore (2004) is due to a divergent approach to concurrent engineering's implementation that mainly focuses on improvement of integration, collaboration and compression of processing with a strong bias on technology while putting little emphasis on identification of value as well as efficiency of the product. In theirconclusion, they argue that in so much as the concurrent engineering tools and mechanism of organizations help to reduce waste resulting to leaner...

Although the importance of defining value and managing flow in Product development has been shown as critical by several authors including (Baines et al., 2006; Browning,2003; Hines, Francis, and Found, 2006), the framework that has been proposed aims to integrate the various techniques and approaches that offer support to the process of optimizing both perspectives at all levels. This framework also tries to incorporate different ways of defining value while at the same time describing techniques that can be used for internal prioritization and ensure coordination is achieved and assured at all levelsused in Lean Product development,
For one to easily understand the different approaches used in the Lean product development suggested by leading researchers in the field of New product development, first and foremost, one needs to do a comprehensive review of the literature on lean design engineering from Bianes et al. (2006). Secondly, one needs to understand the characteristics of the recent best past practice studies that have been carried out in the area of new product development (Cooper, 2008).

Motivation for this research

The efficiency of process must not only be gauged by the level of waster that is reduce or the level of advantages in terms of increase outputs but also with a consideration of other factors such as environmental and sustainability variables (Coelli,2003;2005).It is well-known that lean manufacturing techniques score poorly on environmental variables (Bergmiller,2006).

There is therefore a need to come up with a more elaborate study aimed at determining the 'true' efficiency of the lean manufacturing systems with a consideration of the other important variable of environment and sustainability. This is because there is a general gap in literature in this regard.

Conclusion

There is a need to develop a metric for measuring the efficiency of lean manufacturing but which takes into account the metrics of environment and sustainability