# Step-Down Method
Used when service departments serve other service departments in a one-way chain — no mutual/reciprocal service.
## Procedure
1. Identify the biggest service department and apportion its total cost to all production AND service departments in the given ratio.
2. Then take the second-biggest service department and apportion its (now updated) total cost to all production departments and remaining service departments, EXCEPT the biggest one already closed.
3. Continue step-wise until all service departments are closed.
## How to Spot the "Biggest" Service Department
> In the question, check which department only shows its own cost going out and does NOT receive any apportionment from other service departments — that's the biggest one to apportion first.
## Important Features
- Each service department is closed only once — no come-back.
- It's one-directional flow: no service dept gets cost back after being closed.
- Captures some inter-service relationships but not full reciprocity.