What is aging chain?

An aging chain is a model structure in which items enter the first stock and pass through a sequence of stages, with each stage feeding the next. Typical examples are employees moving from junior to senior, equipment from new to worn, or customers from trial to loyal. It matters because the delay at each stage makes the system respond slowly, and a change at one end takes years to show up at the other.

How it works

Each stage is a stock. The flow out of a stage is often the stock divided by the average time spent in it, so the larger the stock, the bigger the flow. Some items also leave the chain at each stage, through quitting, failure, or churn. The pattern is a series of stock and flow pairs linked end to end.

The chain explains why a hiring burst makes a bulge that moves through the stages over time, and why a hiring freeze creates a gap years later. Because it stores history, it is a source of delay. To track average age, cost, or skill across the chain, add a co-flow. For individual people with different traits, agent-based modeling may be a better fit than lumping them together.

Example

Say a company hires 30 junior staff a year. Juniors stay 3 years before they are promoted to mid-level, and mid-level staff stay 5 years before they become seniors. In steady state juniors number 90 (30 times 3) and mid-level staff number 150 (30 times 5). If hiring doubles to 60 for a year, the junior stock rises quickly, but mid-level numbers only begin to rise in year 3. This is an illustration with round numbers.

How to model it in Machinations

Draw each stage as a Pool and link them with Converters. Set each Converter to move the Pool's content at the rate of one divided by the time in stage, using a Register. Put a Source at the start, a Drain on each stage for leavers, and chart all Pools. Move the hiring Lever and watch the bulge travel.