In manufacturing, few things remain constant and true. Little’s Law is one exception. The equation: Cycle time in days is equal to amount of Work In Process (WIP) in units, divided by the daily output in units is always true and meaningful. If, for example, you have a total of 1000 units throughout the work areas either being worked on, or sitting around, and your output is 100 units per day, your unit cycle time is 10 days long. It is also true, although the equation itself does not specifically point this out, that if you take action that results in a buildup of WIP, and that buildup results in increased cycle time, you have probably made a mistake in judgment.
Continuing on, it is also true that if your WIP remains constant and your output is decreased, your cycle time will go up. If manufacturing could maintain close control over the cycle time of its product, from the time it leaves the release point until it has completely finished, it could safely predict to customers what they could expect in terms of delivery. If the process were completely under control, and nothing bad ever happened, there would be no problem in meeting requirements, if those requirements were within manufacturing’s capabilities. Companies that are able to control their cycle time and meet their requirements are usually very successful manufacturers. They may not be successful in other critical areas, such as sales and planning, however. Sales, production control and executive management can make the best manufacturing processes inept, simply because they often have the power to do so, and just don’t operate from the proper production value set. They make decisions and promises that are destructive to their production processes in order to make their own measurements look favorable. Then, when manufacturing can’t make the promises come true, it is blamed for the failure.
