This document contains questions about system dynamics modeling concepts like stocks, flows, causal loop diagrams, stock and flow diagrams, and reference modes. It asks the reader to identify which variables are stocks and flows in various contexts, draw causal loop and stock and flow diagrams to represent different systems, and build a simple stock and flow model to represent population growth and health sector development over time.

1. A general note: Don’t forget to write the polarity on any of the
causal loop diagrams
Question 1:
1. What do you know about “The snapshot method”?
2. Which of the variables below are stocks and which are flows? Give your answer
in terms of stock and flow diagrams.
a.
b.
c.
d.
e.
f.
g.
h.
i.
Assignment Backlog, Assignment rate, completion rate
oil production, underground oil reservoir, oil storage tank
withdrawals, bank account, deposits
customers leaving, new customers, regular customers
knowledge of words, learning words, forgetting words
population, Births, deaths, net migration rate
self confidence, adjustment of self confidence
employees, hiring, quitting, firing, retiring
product price, adjustment of price
3. Suggest units for all the variables in question 2.
4. Why do you think it matters for an analysis to measure a flow (in or out) in
unit/time?
5. Is it possible for an inflow and outflow to have different unit/time? (for example,
inflow: Km/second and outflow:Km/minute)
Questions 2:
1. Why do you think stocks are critical in generating the dynamics of systems?
(At least three reasons to be mentioned).
2. Talk about causation versus correlation (you can explain through an example).
3. Graphical integration Q
Question 3:
1. You have a bucket completely filled with water and you need to empty the water
till a desired level:
a. Draw a causal loop diagram that captures this process of emptying the bucket
of water till a certain level.
b. Draw a stock and flow diagram that represents this process of filling the
bucket.
c. Can you draw a graph representing the behavior of the stock and flow diagram
you drew in b (N.B.: you’ll empty the bucket completely).
d. For what values do you expect your Inflow to be zero?

2. 2. Build a stock and flow model to give the following structures:
a. Exponential growth
b. Goal seeking behavior
c. Exponential decay
3. You are a modeler considering the population growth issue in country “X”, the
total population of this country increased from 25 million to 80 million persons in
50 years. You divided the population into three cohorts:
- Children: are born and grow up to be young adults.
- Young adults (who are reproductive and increase the number of children) and
then grow up to be adults.
- Adults (who wouldn’t contribute to the increase of population).
The populations of children, young adults are stable and only adults die.
a. Draw a reference mode for the three population cohorts and the total
population.
b. Draw a simple a simple stock and flow diagram to represent this population
issue under analysis.
c. What if the assumption of “only adults die” changes (i.e. children and young
adults die too)…what structure can be added to your model in b to represent
this?
Question 4:
T21 is an integrated analytical tool that can support wide range of analysis and
produces a wide number of indicators. T21 can be very useful in supporting the
preparation of national budgets, national development strategies, poverty
reduction strategies, applications for grants and loans…etc.
1. From what you know about T21, Mention Three characteristics for it.
2. T21 has a higher level structural view to each country; dividing it to 3 main
sectors: economical, environmental and social sectors, where these sectors are
integrated together through feedback relations…Can you give an example of a
feedback loop connecting these three sectors together? (i.e. a feedback loop,
where its different elements belong to these three different sectors)
3. You are building a very simple health sector for country “Z”, the purpose of
that basic structure is to trace the number of health sectors ready to serve
people in the country. And later on it can be used for further modeling
purposes. In year 1985, the number of health sectors in the country were 90
health centers, a health center has an average life of 20 years. The health
centers average construction cost is 20,000,000 money units. The budget that
was dedicated to building these health sectors by the government was
200,000,000 money units.
a. Show the stock and flow structure that you’ll build for this issue.
b. Write the equations you’ll use in your model.
c. Show the units for your model elements.