How to set the product family and value stream boundaries

Why it matters

When preparing a value stream map, the first decision is which products will be examined together. If the family boundary is unclear, stock, time and demand cannot be tied to the same denominator. Two products sold under the same name may pass through different processes; two products with different names may use the same operation path. The color, size or appearance of a product can be helpful information for the family, but it does not replace path information. A family is an analytical boundary set up to solve a particular flow question. This boundary does not have to assign products to physically separate machines. Shared resources may remain; what matters is not to make the load on those resources and the interaction between families invisible. A good boundary says clearly what the map explains and what it leaves out.

A product-process matrix shows products in the rows and processes in the columns. A cell indicates whether the product visits that process; the sequence information is written separately or kept in the cell as a sequence number. Comparing only the marked cells is not enough. Products that pass through the same processes in a different order can behave differently in terms of intermediate stock and information flow. If there is a return to the same process more than once, the repeat visits are recorded separately. The ordered common step ratio used in this lesson is not a universal industry standard or a mandatory acceptance criterion. This local common path ratio is defined, for paths without repeat visits, as the length of the longest sequence of common steps that preserves the same order, divided by the total number of distinct processes used by the products being compared. This explicit definition shows which similarity the calculation results measure. The exact path match ratio in the activity and the module test, on the other hand, is the ratio of the number of products that conform to the whole reference path to the number of candidate products; in its demand-weighted form, the demand of the matching products is divided by the total candidate demand. The two measures are not used in place of each other.

The common path ratio is decision support; it is not a universal acceptance threshold valid for every business and every problem. When a high ratio is found, the times, batch conditions, quality requirements and demand variability are still examined. Even if the common path is the same, one product's long setup can delay the replenishment of the others. A wider family can make the common flow visible, but it can hide the branches that split off and the different stock rates in a single average. A narrower family gives a cleaner calculation but can lead to forgetting competition for shared resources. For this reason at least two boundaries are evaluated. It should be explained which question the chosen boundary answers better, which detail it requires tracking separately, and when it will be reviewed again.

The family decision is not made only for ease of drawing. It determines the denominator of the stock days on the map, the product scope of the kanban loop and the path represented by the measured time. When the boundary changes, no direct comparison is made with the old totals. Demand, stock and processing data are closed again for the new boundary. If a shared machine also works for products left outside, that time enters the capacity calculation; being outside the family does not mean not using the resource. At the end of this lesson the aim is not to find the one right family name but to be able to make a choice that keeps the numbers in the same scope and preserves the shared constraints. Against the appeal of a similar product name, operation path, sequence and resource evidence are sought together.

Formula