A definition of JIT (Just-in-Time) through its 4 distinctive characteristics
What is a company, called THE JIT COMPANY, without its own proper post on the definition of JIT or Just-in-Time? Particularly in a time in which JIT is often seen as an undesirable way of working, and sometimes even regarded as the cause of recent supply disruptions due to COVID 19 because of its focus on low inventories. But the debate is older even than this. Just-in-Time or JIT is seen as a strategy unfit for high-mix, low volume situations, and the job shops that typically work in these environments. Or JIT pops up in heated debates on push versus pull, and the role MRP should play in managing production and supply. So, how to define JIT? What is Just-in-Time really? How can we provide a definition of JIT that helps in these discussions?
Background to Just-in-Time (JIT)
The concept of Just-in-Time (JIT) as we now know it originated in Japan in the period after World War II, when Kiichiro Toyoda, then president of the Toyota Motor Company, said, “Catch up with America in three years.” In his book (“Toyota Production System: Beyond Large-Scale Production”), Taiichi Ohno describes how he figured out that the difference in productivity between American and Japanese workers must have been something like one to nine and how he wondered how an American worker could exert nine or ten times more physical effort. He concluded that the Japanese were wasting something and that if they could eliminate the waste, productivity could rise by a factor of nine or ten. This idea marked the start of the present Toyota production system (TPS).
Not much later, in the 1950s, there was an extreme depression accompanied by an acute management crisis at Toyota, with big labor disputes and the dismissal of many Toyota employees as a result. It was followed by Kiichiro Toyoda’s resignation. Yet suddenly the outbreak of the Korean war in June 1950 brought about an extraordinary economic demand. Toyota had just reduced their labor force due to a lack of work and they did not feel they could add staff again because of the new orders. Therefore, Toyota sought to sustain a higher volume with a smaller workforce by eliminating as much waste as possible. Under these conditions, Toyota developed the JIT system through trial and error.
So, the aim of the JIT system was to answer to the needs of a changing marketplace in a way that eliminated any possible waste, and – as a result – to be able to do more with less.
The aim of JIT is to answer to the needs of the market in a way that eliminates waste.
Just-in-Time to eliminate waste
Ohno quickly understood that if parts arrive any time prior to their need — not at the precise time needed — waste could not be eliminated. Ohno indicated that “If we can master production of exactly what is needed, when it is needed, then waste (muda), unevenness (mura), and unreasonableness (muri) can be eliminated from the workplace.” He saw it as an extremely rational approach to eliminating waste and as the basic philosophy of the Toyota Production System. It was Toyota’s theme, and his personal theme as well (as described in “Just-in-Time for Today and Tomorrow” by Ohno and Mito).
Shingo wrote that despite many people considering Just-in-Time the most prominent feature of the Toyota production system (TPS), it was and is no more than a strategy to reduce waste. “JIT is only an intermediate goal on the road to improvement,” he wrote in his book on “Non-Stock Production: The Shingo System for Continuous Improvement”.
“JIT is only an intermediate goal on the road to improvement” – Shingo
The benefits sought by JIT are less inventory, shorter throughput times, faster market response, greater flexibility in volume and mix, faster defect detection (leading to better quality), lower storage and logistics cost and more productive space usage, more capacity (by not working on unneeded items now), and lower MRO cost.
Furthermore, JIT also sought to drive kaizen, empower operators, and develop mutual dependence and teamwork.
Just-in-Time: what’s in a name?
“Just in time” was meant by Ohno to describe a situation in which materials do not arrive too early or too late, but “just in time”. “Too early” thereby is relative to the moment the part is needed and Ohno explains that “just in time” is therefore not used in the same way as we would use the expression “exactly on time”. It is more used in the way we say, “just a moment,” meaning “only a little bit”. “Just in time” therefore should be interpreted to mean that it is a problem when parts are delivered too early (see also “Taiichi Ohno’s Workplace Management”).
The true meaning of “just-in-time” is in the “just”. We cannot eradicate waste merely by following an “in-time” policy. Just-in-time means simply “timely” whereas to convey a sense of “exactly at a given time”, one should say “just-on-time”. Concentrating on “exactly on time” might encourage early overproduction, Shingo mentioned.
Just-in-time means producing parts or products in exactly the required quantity — just when they are needed. Hall (in his well-known book “zero inventories”) described JIT as to have only the necessary part at the necessary place at the necessary time, or as one could say: “don’t have anything you aren’t working on”. The theoretical JIT ideal, therefore, is for all materials to be in active use as elements of work in process, never at rest. Perfect JIT creates a continuous flow, a concept that can be operationalized through flow velocity and flow smoothness, on which I wrote before.
So, the purpose of the JIT production system is to maintain a well-attuned, timely correspondence to the market needs by providing goods when and in the amount needed.
“Concentrating on “exactly on time” might encourage early overproduction” – Shingo
A definition of JIT: key distinctive characteristics
Now, up to now, many other methods may claim the same orientation, and subscribe to the same objectives as JIT. So, what sets JIT apart is the way it tries to achieve these objectives. JIT is a distinct method and manner to achieve the goal of just-in-time production. So, what are its distinctive characteristics that make up the definition of JIT? Let’s start with the first three (based upon Sugimori and Cho’s 1977 article titled “Toyota Production System and Kanban System Materialization of Just-in-Time and Respect-for-Human System”):
The first requirement Toyota saw for just-in-time production was to enable all processes to quickly gain accurate knowledge of the timing and quantity of parts required. JIT focuses on the completion of the necessary products at the necessary time through the synchronization of all processes. This was done through the principle of parts withdrawal by the subsequent process and by connecting all processes in reverse order. Concretely it means that only the actual physical withdrawal of an item from its location by a downstream process is seen as the requirement for an item for the upstream process. This is what is often referred to as “pull”. It is important to note that nowhere it is mentioned that this principle only refers to repetitive or anonymous parts in parts supermarkets. It may just as well apply to customer-order specific items in FIFO-lanes positioned between processes. You can read more about pull, the kanban system that materializes it, and its challenges in my earlier posts.
2. One-Piece Flow (OPF)
The second aspect of just-in-time production is that all processes approach the condition where each process can produce only one piece, can convey it one at the time and, in addition, have only one piece in stock between the equipment and the processes. This means that no process is allowed to produce an extra amount and carry a surplus of stock between the processes. In short, all processes are withheld from lot production and conveyance. Please note, JIT strives for one-piece flow; it does not require it. This means you can start with pull without one-piece flow in place, as long as you strive continuously for it.
3. Leveling or Smoothing
The third key element of just-in-time is the leveling of production. If the quantity withdrawn by the downstream process would vary considerably, the upstream processes including those at subcontractors and suppliers would always need to maintain peak capacity or hold excessive inventory. Therefore, the withdrawal of parts by the downstream process needs to be leveled. Here as well, it is important to note that when the original market demand is not leveled, it can be leveled using JIT’s approach to mix-level loading (called heijunka). Non-level demand does not rule out the possibility of JIT; it only requires more heijunka.
Little JIT and Big JIT
The above three distinctive features of just-in-time production typically lead to a massive number of other things that you can also witness at a JIT-based factory. It will lead, for instance, to more process-based layouts, more focus on improved process, equipment and quality control, greater efforts to reduce setup times and reduce lot sizes, highly frequent movement of small transportation batches between processes, and to more multi-process handling and multi-skilling of operators.
Furthermore, as processes are more tightly linked to each other, JIT increases the mutual dependence among workers and makes that workers come to the aid of the one who is in trouble. This overall heightened awareness of problems and causes again leads to more improvement ideas according to Schonberger (“Japanese Manufacturing Techniques: Nine Hidden Lessons in Simplicity”). Hall, in turn, describes the effect of JIT driving people to examine operations in more detail to look for improvement. This quick feedback and fast reinforcement motivates workers, makes them more conscientious and responsible, and pushes teamwork to the shop floor.
Because of the foregoing, Hall introduced the idea of “narrow” or “little” JIT, and “broad” or “big” JIT. Little JIT only refers to the production control and transport aspect of JIT, to have only the necessary material at the necessary place at the necessary time. Big JIT, by contrast, refers to all the activities related to just-in-time production. One can even say that “Big JIT” evolved into what is now known as “Lean” after John Krafcik coined the term “Lean” in his 1988 article, “Triumph of the Lean Production System”.
Adding a 4th key JIT characteristic
The risk of only considering the three earlier mentioned characteristics, or even adding the technical consequential effects as defining for just-in-time production, is that other approaches solely focusing on these more “technical” aspects might be seen as JIT, whereas the more people-related aspects are not considered. I would therefore even go so far as to explicitly add this aspect – related to Toyota’s respect for people principle – as the fourth key characteristic making up the definition of JIT.
4. Respect for People
In Toyota’s system, respect for people is present in all activities. Among others, this aspect aims at increasing the autonomy of team members by entrusting them with greater responsibility and authority, having capable workers actively participating in running and improving their workshops, and allowing them to take part in making improvements. This means that any true JIT system enables operators to operate and be an essential part in the execution of the JIT system, spot flow abnormalities, stop production, and participate in subsequent flow and production control improvement initiatives.
When focusing only on technical aspects, other approaches might too easily be qualified as JIT
In summary, the four distinctive characteristics of pull, one-piece flow, leveling and respect for people should be your guide in developing a JIT system, evaluating your current production system, and even provide a yardstick in discussions on whether a certain approach can be seen as JIT, as originally intended.
And to be clear, there is nothing in JIT’s distinct characteristics that prescribes inventories that are too low to serve demand, that does not allow for its application to high-mix, low-volume or even one-of-a-kind production environments, and that prohibits MRP to be used for planning purposes.