Generate alternative system-level designs and subsystem concepts. Rather than try to pick an early winner, aggressively eliminate alternatives. The designs that survive are your most robust alternatives.
—Allen C. Ward, Lean Product and Process Development
Principle #3 – Assume variability; preserve options
Systems builders tend to have a natural inclination to try to reduce variability. It just seems that the more you think you know and have already decided, the further along you are. But this is often not the case.
While it is true that variability can lead to bad outcomes, the opposite case can also be true.
Variability is not inherently bad or good. Rather, it is the economics associated with the timing and type of variability that determines the outcomes. A focus on eliminating variability too soon perpetuates a risk avoidance culture wherein people can’t make mistakes and gain experience by learning what works and what doesn’t.
Other than a general understanding of system intent, Lean systems builders recognize that very little is actually known at the beginning of the project. If it was, they would have already built it. However, traditional design practices tend to drive developers to quickly converge on a single option—a point in the potential solution space—and then modify that design until it eventually meets the system intent. This can be an effective approach, unless of course one picks the wrong starting point; then subsequent iterations to refine that solution can be very time consuming and lead to a suboptimal design . And the bigger and more technically innovative the system is, the higher the odds are that your starting point was not the optimal one.
A better approach, referred to as Set-Based Design or Set-Based Concurrent Engineering , is illustrated in the figure below.
In this approach, the systems builder initially casts a wide net by considering multiple design choices at the start. Thereafter, they continuously evaluate economic and technical tradeoffs—typically as exhibited by objective evidence presented at integration learning points. They then eliminate the weaker options over time; and finally, converge on a final design, based on the knowledge that has been gained to that point.
This process leaves design options open as long as possible, converges as and when necessary, and produces more optimal technical and economic outcomes.
 Iansiti, Marco. “Shooting the Rapids: Managing Product Development in Turbulent Environments.” California Management Review 38 (1995): 37–58.
 Ward, Allan C. and Durward Sobek. Lean Product and Process Development. Lean Enterprise Institute Inc., 2014.
Last update: 11 May 2015