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Levelization - Heijyun & Heikin -


“Heijyunn-ka.”

Heijyunn-ka, a cornerstone of the Toyota Production System, is fascinating. Its translation, “(Production) Levelization,” is significant for operations management.


In Taiichi Ohno’s “Toyota Production System” (1978, p.25), Ohno described the following;

“Instead, start from the Toyota Motor internally to work out from “End of month push production” into averaging and leveling production.”


The exciting phrase is that Ohno used both “averaging and leveling” in this sentence. I used to think that both have the same meaning. I assume that is why the English version was not translated as “average.” But every word has meanings and stories on why they exist. I took a little dive.


“Averaging” is 【平均化】.

“Leveling” is 【平準化】.





The symbol 【平】 means flat. It is the shape of grass floating on the surface of the water. When the water flows flat, the grass can float. Such a condition is the base of this symbol.







The symbol 【均】 means equal. The left side 【土】 is soil or a block of soil used as an altar of a ceremony. The right side 【勻】 comes from a human palm flattening the surface.







The symbol 【準】 is the complicated one. It comes from a tool used to measure the flatness of flowing water. It was a wooden leveling tool. The tool was called “Mizumori.” Such a tool is used even today to flatten the ground's surface. However, I am unsure if the current method or tool is the same. From the tool, the symbol started to represent the rule or guideline. Therefore, the symbol is used frequently in idioms such as standard 【標準】.


These symbols were combined to form the idioms.





As I wrote before, the idiom Heikin【平均】 is typically translated as average. Yet, that is after mathematics used as such meaning. The original use was equal without difference. Fair. Without “Mura” fluctuation. Also, it was used as “Samurai subjugation of the nation.” It meant the human activity of flattening the variations physically. Taiichi Ohno didn’t like the average cycle time or inventory level. If so, why did he use the word “Heikin” here? But then, the original meaning of the word makes sense. Maybe a better way to translate is “physical leveling.”





The idiom Heijyun 【平準】 has a history.

The idiom was used in a law of the Han dynasty, which aimed to stabilize the price of goods used for tax purposes. The government buys these goods when the price is low and releases them when it is high. It is important to note that this is not price control. It is designed to stabilize the price, almost like the central banks today trying to stabilize inflation. The price still changes. The government functions as the stabilizer, not the controller.


The exciting thing is that this law has a twin. The twin is called Kinyu-hou 【均輸】. The idea of this law was to reduce the workload of taxes. The Han dynasty collected taxes and then sent them to the capital. Under such a law, those living far from the capital had extra transportation work. For them, it was cheaper to buy taxable goods near the capital and pay for those duties. The Kinyu law understood this condition and changed the tax rate according to the transportation distance (or workload). The law divided the nation into zones based on distance and adjusted tax rate. This helped the inflation near the capital.


These laws of the Han dynasty make me think about the 【均】 and 【準】. 【均】 seems to mean material or physical equals. 【準】 appears to be talking about information or rules to level. Heikin 【平均】 means physical leveling, while Heijyun 【平準】 is about information leveling.


If I want the water to flow in a leveled way, I should flatten the ground and supply the water in a leveled way.


And “good” legalization requires both.


In today’s manufacturing, we see many “mixed model lines.” These lines require different components, and massive stores are opened randomly. This is where operators search for parts, and violations of the 5S occur. But how can it be said that “mixed model lines” operate with current physical conditions? I am not saying it will require more space since that is also impossible. Then, we must think differently about transportation methods, just like the Han dynasty. For example, should we keep delivering the containers with a forklift? The quantity transported will be huge, which could last days or weeks. Why do we need to keep such a large quantity? Why can’t we mix and transport the parts to satisfy the “mixed model line”?


There are more physical requirements for “good” leveling. Yet, instead of working on those requirements, we see implementing the Heijyun-ka box or forcing the hard work on the shop floor. Good leveling should make the job easier and more stable. As one plant manager who implemented the leveling said, “calm state.” It shouldn’t come from operators’ hard work figuring out things.


Balance of physical and informational leveling. How are your operations taking care of these?

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