Stock and flow diagram

• • • IN CONSTRUCTION --Tomyk007 21:12, 22 January 2014 (CET)

Definition

A Stock & Flow Diagram (SFD) is similar to a Causal Loops Diagram (CLD) though provides a more rigourous representation. By explicitly identifying Stocks, reservoirs which change over time, and Flows, which are responsible for changes in Stocks, the resultant insight explicitly depicts the manner in which things change.

Captures

• Stock
• Flows
• Quatified feedback loops

Rules

• Stocks are always changed by flows
• In general, the formulas for the flows will depend on things that are changing (state)

Stock Flow Symbols

State of the System: Stocks (“Levels”, “State Variables”, “Compartments”) [x]

• Stocks (Levels) represent accumulations

– These capture the “state of the system” – Mathematically, we will call these “state variables” • These can be measured at one instant in time • Stocks start with some initial value & are thereafter changed only by flows into & out of them – There are no inputs that immediately change stocks • Stocks are the source of delay in a system • In a stock & flow diagram, shown as rectangles

Stocks (also known as levels, accumulations, or state variables) are used to represent the real-world processes (e.g. stocks of material, knowledge, people, money). They change their value continuously over time with the given flows.

Stocks are drawn with rectangles that represent some accumulators containing some amount of contents of the stock.[y]

Examples of Stocks

There is list of common examples of Stocks [x]:

• Water in a tub or reservoir
• People of different types
  • – {Susceptible, infective,immune} people
  • – Pregnant women
  • – Women between the age of x and y
  • – High-risk individuals
• Healthcare workers
• Medicine in stocks
• Money in bank account
• CO2 in atmosphere
• Blood sugar
• Stored Energy
• Degree of belief in X
• Stockpiled vaccines
• Goods in a warehouse
• Beds in an emergency room
• Owned vehicles

Examples of Flows

There is list of common examples of Flows [x]:

• Inflow or outflow of a bathtub (litres/minute)
• Rate of incident cases (e.g. people/month)
• Rate of recovery
• Rate of mortality (e.g. people/year)
• Rate of births (e.g. babies/year)
• Rate of treatment (people/day)
• Rate of caloric consumption (kcal/day)
• Rate of pregnancies (pregnancies/month)
• Reactivation Rate (# of TB cases reactivating per unit time)
• Revenue ($/month)
• Spending rate ($/month)
• Power (Watts)
• Rate of energy expenditure
• Vehicle sales
• Vaccine sales
• Shipping rate of goods

Software tools

• Vensim
• Powersim
• IThink/Stella

Examples

A manufacturing firm

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.

References [x] http://www.cs.usask.ca/~osgood/Classes/CMPT858Spring2011/LectureSlides/Lecture%207%20--%20Stock%20and%20Flows.pdf [y] http://www.anylogic.com/anylogic/help/index.jsp?topic=/com.xj.anylogic.help/html/sd/Stock%20and%20Flow%20Diagrams.html