Page 29 - MOH Supply and Needs-Based Requirement Hyperlink 02032021

21 EXTENDED EXECUTIVE SUMMARY

Fundamentally, SD deals with stocks and flows, internal feedback loops, and time delays
to represent the structure of the system that affects system behaviour that changes
dynamically over time. Stocks and flows are used to describe the units flowing through
the system with flows affecting the stock levels. A “stock” is a quantity that either grows
or depletes over time, a “flow” is a connector that drives the increase or decrease of a
stock. In summary, three key concepts in system dynamics approach System Thinking,
Feedback Loop, and Stock and Flow. Two key SD diagramming tools are used to assist
in translating system structure into the model structure, the Causal Loop Diagram
(CLD) and Stock and Flow Diagram (SFD), as illustrated Figure 7 in and Figure 8 .

Figure 7: The Causal Loop Diagram for Basic Registered Doctor Supply

CLD

+ +

New doctors rate Registered
(local grads) R Doctors B Attrition +
+ +
+ -
R
New doctors Attrition rate
(local grads)
+
+ New doctors
(overseas grads)

The CLD in Figure 7 illustrates a simple doctor supply system. The increase in the number
of medical graduates (both from local and overseas) increases the number of registered
doctors available. The increase in the number of attritions, on the other hand, decreases
the number of registered doctors.

While the advantage of CLD is the simplicity of its diagrammatic conventions for
representing and communicating the major circular causality mechanism, this simplicity
is a limitation because it does not distinguish between the different types of variables such
as stocks, flows and information. SD then uses SFD as the basis for building simulation
models for quantitative analyses.

SUPPLY AND NEEDS-BASED REQUIREMENT PROJECTIONS OF MALAYSIAN HUMAN
RESOURCES FOR HEALTH USING SYSTEM DYNAMICS APPROACH 2016 - 2030