An Empirical Study of WIP in Kanban Teams
https://www.sintef.no/en/publications/publication/1637884/
Method
"A quantitative case study was conducted in a software company that provided information about more than 8,000 work items developed over four years by five teams. Relationships between WIP, lead time and productivity were analyzed."
Population
"more than 8,000 work items developed over four years by five teams"
What it does not show
The same data complicates the simple story: “WIP also correlates with productivity, which is inconsistent with the claim in the literature that a low WIP… will improve productivity”, and the author concludes an optimal WIP limit was difficult to identify. Quality could not be assessed at all — the dataset lacked the information. Single company; the author explicitly calls for replication.
Dag I. K. Sjøberg
“WIP correlates with lead time; that is, lower WIP indicates shorter lead times, which is consistent with claims in the literature” — direct production-data support for the queueing mechanism the slack argument rests on.
Tier II: Observational quantitative case study inside one company correlating work-in-progress against lead time and productivity over 8,000+ real work items. No manipulation of WIP and no control group.