This was inspired by a thread on Jim Shore's excellent blog. You should probably start there.
Where I work, planned MMFs (as opposed to emergent support items, or dev-initiated work) are pretty chunky: usually several pair-days. Moreover, since we do weekly meetings with the customer to re-assess the queue, and our throughput is much slower than a week (usually 30+ calendar days), items at the bottom can stagnate due to multiple unfavorable re-assessments. Our median (for completed work, whether MMF or no) is much better than our average (for queued MMFs). In practice this means the queue is FIFO/LIELO (First In First Out/Later In Even Later Out).
Indeed, you could make the argument that our queue is not so much a queue as it is a kind of event horizon in reverse. Hey! I'll make that argument.
The event horizon of a black hole is a characteristic distance, such that anything that comes inside its radius gets gravitated in, even radiation, never to be seen again outside the EH. In the case of our planning queue, it's not about the attraction of gravity so much as the repulsive (repellent? there's no upbeat way to say it, and maybe there shouldn't be) force on a task due to other tasks competing their way ahead more effectively. The REH (Reverse Event Horizon) is a number that represents the point in the queue such that, if your task is always that far back, its expected wait time is infinite. The repulsive force is (expected to be) insurmountable.
For us, "constraining" the queue to 7 slots has been anecdotally beneficial, but I suspect the benefit does not especially come from imposing structured flow on development. There's still a considerable amount of chaos in our flow. And, we can't conclude anything more than "anecdotal" benefit because we, erm, have metrics issues that I won't get into here. Indeed, I'm not sure Kanban has changed our development flow at all.
Instead, our benefit comes from not having to plan/estimate/presupport (lose a large amount of our finite attentiveness to) tasks that will never get done, or that will have to be radically re-imagined by the time we are ready to work on them. The information flow has been smoothed, even if the work product turbulence is about where it was before Kanban.
It's not just that the information flow is smoother on a per-task basis, either. I think Kanban has also reduced the baseline difficulty of planning. In particular, I think that once customers began to think of tasks as inventory (and maybe especially as things that both tie up "capital" and lose value over time), they began to give themselves permission to plan less. Or at least, plan fewer: each thing can still get the careful consideration it used to get, but fewer things enter the conversation. I think we all know there's a cost to planning, but Kanban makes its costs more familiar and apparent.
Next post, I'll look at actual numbers to argue that 7 items is a pretty good queue depth for us. Teaser: My guess is that our actual REH is somewhere around 10 or 12. I know that we finish 1-3 queued items per planning iteration.
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