Kanban and Limiting Work-in-Progress
There is causation between quantity of work-in-progress and average lead time, and the relationship is linear. In the manufacturing industry, this relationship is known as Little's Law. The evidence from these two teams at Motorola suggests that there is a correlation between increased lead time and poorer quality. Longer lead times seem to be associated with significantly poorer quality. In fact, an approximately six-and-a-half times increase in average lead time resulted in a greater than 30-fold increase in initial defects. Longer average lead times result from greater amounts of work-in-progress. Hence, the management leverage point for improving quality is to reduce the quantity of work-in-progress. Since uncovering this evidence I have managed work-in-progress as a means to control quality and have become convinced of the relationship between the quantity of WIP and initial quality. However, at the time of writing [i.e., circa 2011] there is no scientific evidence to back up this empirically observed result.
Reducing work-in-progress, or shortening the length of an iteration, will have a significant impact on initial quality. It appears that the relationship between quantity of work-in-progress and initial quality is non-linear; that is, defects will rise disproportionately to increases in quantity of WIP. Therefore, it makes sense that two-week interations are better than four-week iterations and that one week iterations are better still. Shorter iterations will drive higher quality.
-- David J. Anderson
Quoted on Mon Jul 30th, 2012