Mapping the Stock and Flow Structure of Systems$*1$

Assigned: Monday March $12$th$,2024$; Due: $11$:$59$PM$,$ Monday March $18$th$,2022.$ Submit on Blackboard under ST assignment$-1$

This assignment gives you practice with the structure and dynamics of stocks and flows. Stocks and flows are the building blocks from which every dynamic system is constructed. The ability to identify, map, and understand the dynamics of the networks of stocks and flows in a system is essential to understanding the processes of interest in any modeling effort.

A$.$ Identifying Stock and Flow Variables

The distinction between stocks and flows is crucial for understanding the source of dynamics. In physical systems it is usually obvious which variables are stocks and which are flows. In human and social systems, often characterized by intangible, $$soft$$ variables, identification may be more difficult. For each of the following variables, state whether it is a stock or a flow, and give units of measure for each.

Name Type

Example: Inventory of beer Stock Example: Beer order rate Flow Preorders for iPhone $13$

Koc Tuition

Tesla Model $3$ Production

Google rating of your business Depreciation

Global production of solar power

Accounts payable

Promotion of junior partners to full partners at a consulting firm

International Energy Agency forecast of global solar power

A firm$$s cost of goods sold $($COGS$)$ Customers complaining about long wait times for call center service

Fenerbah$$e wins to date this season Turkey current account deficit

CO$2$ in the atmosphere

Mapping Stock and Flow Networks $(3$ points$)$

Units

Cases Cases$/$week

B$.$

Systems are composed of interconnected networks of stocks and flows. Modelers must be able to represent the stock and flow networks of people, material, goods, money, energy, etc. from which systems are built.

$*1-$ Based on Assignments developed. By John Sterman and Nelson Repenning of MIT Sloan School of Management.

For the following case, construct a stock and flow diagram that properly maps the stock and flow networks described.

$*$ Not all the variables are connected by physical flows; they may be linked by information flows, as in the example in the next page.

$*$ You may need to add additional stocks or flows beyond those specified to complete your diagram $($but keep it simple$).$ Be sure to consider the boundary of your stock and flow map.That is$,$ what are the sources and sinks for the stock and flow networks? Are you tracking sources and sinks far enough upstream and downstream? This process of deciding how far to extend the stock and flow network is called $$challenging the clouds$$ because you question whether the clouds are in fact unlimited sources or sinks$.$

$*$ Consider the units of measure for your variables and make sure they are consistent withineach stock and flow chain.

Assume the client is the Istanbul Airport Security Administration $($IASA from now on$)$ and that they are interested in improving the quality and efficiency of the screening process at theairport. Map the process of going through security at the airport. While the configuration may differ based on timing and location, consider a process with the following steps:

$$ Customers must have their tickets and identification documents checked by a IASA agent before entering the scanning phase.

$$ Passengers must surrender all metal items$$phones etc.$$prior to entering the body scanner.

$$ The security process in question uses body scanning technology which requires approximately ten seconds to complete.

$$ After the scan is complete, the traveler must wait until IASA has either confirmed thatnothing was identified or that the traveler requires additional screening.

$$ If the body scan does identify something, then additional screening is required.

$$ Assume that the additional screening is definitive in the sense that following it passengers are either allowed to enter the secure area or must surrender whatever item is considered to be illegal.

$$ Note the final stages of the process differ depending on whether or not the scanner yields a positive image. Make sure you capture the stocks and flows associated with both outcomes.

$$ For simplicity, you do not have to create a separate stock$-$flow structure for luggage. That said, do account for the fact that passengers may spend time waiting to surrender and retrieve it$.$

$$ Accounting for luggage explicitly is a bit subtle. Try to add this in a separate diagram if you want to stretch your skills $($your answer to this part will not be part of your grade$).$

Example: A manufacturing firm maintains an inventory of finished goods from which it ships to customers. Customer orders are filled after a delay caused by order processing, credit checks, etc. Map the stock and flow structure, drawing on the following variables: Inventory, Raw Materials, Production, Order Backlog, Order Rate.

Solution:

The unit of measure in this flow is widgets $/$ time period.

Raw Materials

Inventory