Chris and Adams co-author a book and now visiting different countries to market their book. It is the last stop for their journey to attend a seminar in Chris's country, and Adam will travel back to his homeland after that. When they reached the airport, they realized that their flight is canceled due to technical difficulties. However, the airline arranged a flight via another partner company that causes an extra stopover and a few more hours to reach the destination. To compensate for this unnecessary trouble, the airline upgraded both to business class, not a bad deal. When they reached their destination, Chris, being a citizen of the country, came to the baggage claim area much faster than Adam.
What is unique in this short story of Chris and Adam? This story demonstrates the concept of “class of service” and “work item type.” Explore both one by one. Class of service is not a new concept Don Reinertsen discussed this as principle W18 “The Principle of Differential Service” in his famous book . David Anderson defines the class of service with the following words “class of service provides us a convenient way of classifying work to provide acceptable levels of customer satisfaction at an economically optimal cost.“  Mike Burrows defines it as “customer expectations defined broadly for different subsets of the overall workload” .
There are four commonly used classes of services defined in the Kanban literature as “expedite,” “fixed date,” “standard,” and “intangible.” Without going into much detail about all of this, it is important to remember that based on the situation and context class of service can be changed; typical examples are intangible may become a fixed date, or a fixed date may become an expedite.
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Look at the story of Chris and Adam. They are using the service of an airline to travel from one location to another. From the airline point of view work item type for all the passengers is the same; all of them are traveling from one location to another. However, the airline is providing different classes of services to passengers depends on their tickets, such as economy, business, and first-class. Depends on the situation, they can upgrade the passenger to a different class, such as frequent flyer points. In the above case, Chris and Adam's class of service upgraded from economy to business class.
However, when they landed at the airport, they went to a different queue depends on their citizenship and faced different steps to pass the immigration. From the airport point of view, they provide the services to passengers to do immigration compliance with the land law. For them, Chris and Adams are two different work item types and need to be treated differently. Unlike the class of service, they cannot change the work item type based on the context.
Here is another example; the software build on the old operating system whose support will be ended in 3 years. It may be an intangible class now; however, it may become a fixed date and eventually expedite the closer we get to the support end date. However, that software needs custom hardware build by the company, which is an entirely different work item type than software and can not change like a class of service.
Similarly, there is an option on some roads to drive in the toll lane to travel faster if the driver is willing to pay extra money. It is an example of a different class of service based on the toll lane and regular lane. On any particular day, one can pay extra money to get a different class of service. However, not all roads allow the vehicle with hazardous stuff. It is a different work item type and can not be changed by just paying some extra money.
It is important to note that different work item types can have the same class of service, just like Chris and Adam both are traveling to the business class or passenger car and truck with hazardous material both are on the toll road. Besides, the same work item type may have a different class of service, such as passengers from the same citizenship may travel in economy and business class or passenger cars used regular and toll lane.
Know More: Team Kanban Practitioner
The Principles of Product Development Flow by Donald G. Reinertsen
Kanban by David J. Anderson
Kanban from the Inside by Mike Burrows
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Scrum.org introduced 4 basic key flow metrics in “The Kanban Guide for Scrum Team” named “Work in progress”, “Cycle time”, “Work item age” and “Throughput”....