Horizontal Dependency
Definition
Horizontal dependency is the relationship among components at the same level of a bill of material: because all of them are required to build one parent unit, a shortage of any single component blocks the assembly of all the others.
In Practice
Horizontal dependency is why component availability must be planned as a set. If a kit needs 40 parts and 39 are on hand, the assembly still cannot start, and service level compounds multiplicatively: with each of 40 components independently available 98% of the time, the odds of a complete kit are roughly 0.98^40, about 45%.
Planners manage horizontal dependency through kitting checks before order release, allocating scarce components to specific orders, and setting component safety stocks with the full-kit probability in mind rather than item-by-item.
Frequently Asked Questions
What is the difference between horizontal and vertical dependency?
Horizontal dependency links components at the same BOM level. All siblings are needed simultaneously to build the parent. Vertical dependency links levels: a parent's order generates demand for its components below. MRP explodes vertical dependencies; shortage checking and kitting manage horizontal ones.
Why does horizontal dependency hurt service levels?
Availability multiplies across required components. Even with each of 20 parts at 99% availability, the chance of having all 20 at once is about 82%. That is why assembly operations verify full-kit availability before releasing orders, and why one cheap C-item can idle an expensive line.
Related Terms
Vertical dependency is the parent-child relationship between levels of a bill of material: demand for a parent item directly creates demand for the components beneath it, cascading down through every BOM level.
KitA kit is a grouped set of components picked and issued together as one unit (either staged for a production order or sold as a bundled item) so everything needed for assembly or use arrives complete.
Material Requirements Planning (MRP)Material Requirements Planning (MRP) is a calculation engine that translates a production schedule into time-phased requirements for every component and raw material, using bills of materials, inventory records, and lead times.
Planning Bill of MaterialA planning bill of material is an artificial BOM that groups items for forecasting rather than building, typically a product family parent whose components are options and common parts weighted by their expected percentage of total demand.