System Dynamics

Understanding Why Systems Overshoot, Oscillate, and Fail to Recover - The Key is Feedback and Time Delays

System Dynamics doesn't focus on individual elements, but on how "stocks accumulate", "flows change stocks", and "feedback loops amplify or dampen changes". Many seemingly counterintuitive phenomena are essentially the result of feedback and time delays working together.

Stocks Flows Feedback Loops Delays & Oscillations
Enter Interactive Lab

System Dynamics Perspective

Problems often don't occur at a single moment, but stem from the structure itself: what accumulates, what flows, what delays, what feeds back.

Stocks Stocks are accumulated quantities, such as inventory, work-in-progress, backorders, cash, and population.
Flows Production, shipments, replenishment, births, deaths are all flows that change the level of stocks.
Feedback Low stock stimulates replenishment, high stock suppresses it - this is a typical balancing loop.
Time Delay When there's a delay between decisions and outcomes, systems easily overshoot, oscillate, and overcorrect.
Understanding the Method

The Key to System Dynamics: How Structure Drives Outcomes Over Time

This page demonstrates System Dynamics using an "inventory-in-transit-backlog" supply-demand system. It's perfect for understanding why real-world replenishment systems tend to overreact.

1

Identify Stocks First

Stocks are not "instantaneous variables" but accumulated state quantities. System Dynamics always focuses on these stocks first.

2

Then Identify Flows

Flows determine how stocks change. For example, production flows into inventory, shipments flow out, and demand gaps flow into backlog.

3

Identify Feedback Loops

Declining inventory drives replenishment, which in turn changes inventory levels; these closed-loop relationships are the root cause of system behavior.

4

Watch for Delays

If there are delays in production and transportation, today's decisions may take days to show effects, making the system prone to oscillations.

Teaching Experiment on This Page

We simulate an inventory system: after demand suddenly rises, replenishment decisions take production delays to arrive. You can drag parameters to directly observe overshoot, backlog, and recovery processes.

Interactive Lab

Adjust Target Inventory and Response Speed to See Why Systems Oscillate

The main diagram below shows the inventory structure at the selected time. You can also drag "View Day" to see the internal state of the same system at different time slices.

Inventory-Flow Structure at Current Time
Stop time at a specific day, and you'll see how inventory, in-transit, and backlog connect through flows. This is the core view of System Dynamics.
Inventory / In-Transit Backlog Decisions & Flows
Temporal Evolution Curve
If you see significant inventory overshoot, backlog rising then falling, and replenishment continuously lagging, this is the typical result of feedback and time delays working together.
How to Read Results
The most important value of System Dynamics is explaining "why it happens" as structures and loops, rather than just describing the surface results.

Waiting for simulation results...

When to Use

When the Question is "How Will the System React?" Not "What Will Someone Do?"

Inventory replenishment, capacity expansion, population growth, resource consumption, learning curves, and organizational inertia are all well-suited for understanding from the perspective of stocks, flows, feedback, and delays.

①

Accumulations Exist in the System

If the system contains accumulating state variables like inventory, cash, population, skills, or in-transit orders, System Dynamics is very useful.

②

Strong Feedback Loops

When a state change feeds back to influence its own future rate of change, this is the closed-loop structure that System Dynamics excels at handling.

③

Delays Amplify Problems

If today's decisions take days to take effect, systems tend to overshoot, oscillate, and lag in recovery—System Dynamics is ideal for explaining this phenomenon.