I was recently in a discussion where I was asked to explain the difference between Agile, Lean and Six Sigma. So I thought it would be worth sharing my (expanded) response here.
Starting with Six Sigma we are talking about a set of techniques and tools for process improvement. In particular the emphasis is on using statistical tools and quality management techniques to focus these improvements on the primary causes of defects and variability in a process.
Lean, on the other hand, is focused on preserving value and the elimination of waste. There are seven forms of waste that must be eliminated or, at least, reduced: waiting, overproduction, transport (risk of damage/loss/delay), over-processing (gold-plating), inventory (frozen capital), motion (wear & tear), defects (impact of rework). In contrast to the quality focus of Six Sigma, Lean puts a stronger emphasis on speed and deals with issues of flow.
Hybridised approaches exist and Lean Six Sigma, for example, targets flow, speed, defects and variability. Quality is customer-centric and decisions are data-driven.
Agile is not directly concerned with process improvement. It is an iterative and incremental approach to software development that adopts rapid release cycles to repeatedly deliver small but complete segments of the final solution. The focus is on prioritised feature development and quick deployment among the user community to facilitate refinement of both the remaining requirements and future direction. Agile borrows many ideas from Lean Manufacturing that relate to the management of inventory and flow.
And then there’s Lean Startup where the biggest form of waste is developing something that nobody wants whether it be the whole product/service or just unnecessary features, distribution channels or whatever. Lean Startup emphasises the ideas of end-user engagement, early and continual validation of prototype solutions and adaptation through learning. There is a significant parallel with Agile (thanks to the common Lean genesis) but Lean Startup widens the scope beyond software to examine the broader business canvass.
In my view all of these approaches are complimentary and contain something pertinent to just about any endeavour. As is often the case, a hybrid approach tailored to the context is likely to be the best option.