The idea
An increase in cumulative enrollments can coexist with a stagnant active population. Net change depends on both entries and exits. Following only the inflow can make a system appear healthy while accumulated losses quietly offset the gains.
Worked example
A fictional learning group recruits 40 members each month. Its public report celebrates 480 signups over a year. Yet about 35 active members leave every month, so the active group grows much more slowly than the signup count suggests. The coordinator records current active membership, new entries, departures, and reactivations separately. This creates a more useful question: what keeps members engaged, and which departures reflect normal completion rather than dissatisfaction?
Use it carefully
An exit is not automatically a failure. Learners may finish a course, move away, or shift to another service. Define the stock according to the decision being made, and avoid changing “active” merely to improve a chart. Distinguish unique people from accounts and specify the observation window. Net accounting reveals the arithmetic, while explaining the flows still requires evidence about behavior.
FROM UNDERSTANDING TO ACTION
Put it to work.
Create a one-month membership balance with entries, exits, and reactivations. Explain why cumulative signups answer a different question.
RETRIEVE. REASON. REMEMBER.
Knowledge check.
Choose one answer for each question, then check your reasoning.
Sources & further reading.
Links support the core concepts or identify the book. Examples, exercises, and critical analysis are original Institute material. Source checks are not an independent expert review of every claim.
- MIT System Dynamics: stock changes as inflows less outflows ↗
MIT System Dynamics: stock changes as inflows less outflows; supports the core concept. Scenarios, calculations, and practice questions are original. Source reviewed 2026-10-19.