9

BATCH

In the book Lean Thinking, James Womack and Daniel Jones recount a story of stuffing newsletters into envelopes with the assistance of one of the author’s two young children. Every envelope had to be addressed, stamped, filled with a letter, and sealed. The daughters, age six and nine, knew how they should go about completing the project: “Daddy, first you should fold all of the newsletters. Then you should attach the seal. Then you should put on the stamps.” Their father wanted to do it the counterintuitive way: complete each envelope one at a time. They—like most of us—thought that was backward, explaining to him “that wouldn’t be efficient!” He and his daughters each took half the envelopes and competed to see who would finish first.

The father won the race, and not just because he is an adult. It happened because the one envelope at a time approach is a faster way of getting the job done even though it seems inefficient. This has been confirmed in many studies, including one that was recorded on video.¹

The one envelope at a time approach is called “single-piece flow” in lean manufacturing. It works because of the surprising power of small batches. When we do work that proceeds in stages, the “batch size” refers to how much work moves from one stage to the next at a time. For example, if we were stuffing one hundred envelopes, the intuitive way to do it—folding one hundred letters at a time—would have a batch size of one hundred. Single-piece flow is so named because it has a batch size of one.

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Why does stuffing one envelope at a time get the job done faster even though it seems like it would be slower? Because our intuition doesn’t take into account the extra time required to sort, stack, and move around the large piles of half-complete envelopes when it’s done the other way.² It seems more efficient to repeat the same task over and over, in part because we expect that we will get better at this simple task the more we do it. Unfortunately, in process-oriented work like this, individual performance is not nearly as important as the overall performance of the system.

Even if the amount of time that each process took was exactly the same, the small batch production approach still would be superior, and for even more counterintuitive reasons. For example, imagine that the letters didn’t fit in the envelopes. With the large-batch approach, we wouldn’t find that out until nearly the end. With small batches, we’d know almost immediately. What if the envelopes are defective and won’t seal? In the large-batch approach, we’d have to unstuff all the envelopes, get new ones, and restuff them. In the small-batch approach, we’d find this out immediately and have no rework required.

All these issues are visible in a process as simple as stuffing envelopes, but they are of real and much greater consequence in the work of every company, large or small. The small-batch approach produces a finished product every few seconds, whereas the large-batch approach must deliver all the products at once, at the end. Imagine what this might look like if the time horizon was hours, days, or weeks. What if it turns out that the customers have decided they don’t want the product? Which process would allow a company to find this out sooner?

Lean manufacturers discovered the benefits of small batches decades ago. In the post–World War II economy, Japanese carmakers such as Toyota could not compete with huge American factories that used the latest mass production techniques. Following the intuitively efficient way of building, mass production factories built cars by using ever-larger batch sizes. They would spend huge amounts of money buying machines that could produce car parts by the tens, hundreds, or thousands. By keeping those machines running at peak speed, they could drive down the unit cost of each part and produce cars that were incredibly inexpensive so long as they were completely uniform.

The Japanese car market was far too small for companies such as Toyota to employ those economies of scale; thus, Japanese companies faced intense pressure from mass production. Also, in the war-ravaged Japanese economy, capital was not available for massive investments in large machines.

It was against this backdrop that innovators such as Taiichi Ohno, Shigeo Shingo, and others found a way to succeed by using small batches. Instead of buying large specialized machines that could produce thousands of parts at a time, Toyota used smaller general-purpose machines that could produce a wide variety of parts in small batches. This required figuring out ways to reconfigure each machine rapidly to make the right part at the right time. By focusing on this “changeover time,” Toyota was able to produce entire automobiles by using small batches throughout the process.

This rapid changing of machines was no easy feat. As in any lean transformation, existing systems and tools often need to be reinvented to support working in smaller batches. Shigeo Shingo created the concept of SMED (Single-Minute Exchange of Die) in order to enable a smaller batch size of work in early Toyota factories. He was so relentless in rethinking the way machines were operated that he was able to reduce changeover times that previously took hours to less than ten minutes. He did this, not by asking workers to work faster, but by reimagining and restructuring the work that needed to be done. Every investment in better tools and process had a corresponding benefit in terms of shrinking the batch size of work.

Because of its smaller batch size, Toyota was able to produce a much greater diversity of products. It was no longer necessary that each product be exactly the same to gain the economies of scale that powered mass production. Thus, Toyota could serve its smaller, more fragmented markets and still compete with the mass producers. Over time, that capability allowed Toyota to move successfully into larger and larger markets until it became the world’s largest automaker in 2008.

The biggest advantage of working in small batches is that quality problems can be identified much sooner. This is the origin of Toyota’s famous andon cord, which allows any worker to ask for help as soon as they notice any problem, such as a defect in a physical part, stopping the entire production line if it cannot be corrected immediately. This is another very counterintuitive practice. An assembly line works best when it is functioning smoothly, rolling car after car off the end of the line. The andon cord can interrupt this careful flow as the line is halted repeatedly. However, the benefits of finding and fixing problems faster outweigh this cost. This process of continuously driving out defects has been a win-win for Toyota and its customers. It is the root cause of Toyota’s historic high quality ratings and low costs.