Marzia Ghisotti Thann Thao Tran Valeria Vanelli

Simulation models for economics

Project work on

"Time bank."


The applet requires Java 1.4.1 or higher. It will not run on Windows 95 or Mac OS 8 or 9. Mac users must have OS X 10.2.6 or higher and use a browser that supports Java 1.4. (Safari works, IE does not. Mac OS X comes with Safari. Open Safari and set it as your default web browser under Safari/Preferences/General.) On other operating systems, you may obtain the latest Java plugin from Sun's Java site.

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view/download model file: time_bank.nlogo


The model simulates the functioning of the "Time Bank". This institution is not a real bank, it is an association which allow people to exchange hours of work using units of time as currency and is an example of an alternative economic system. Essentially, the "time" one spends providing these types of community services earns "time" that one can spend to receive services. Therefore no money is transferred among partecipants, who have only to pay an annual membership fee to the "bank".
Our aim is to monitor the behaviour of members, also when opportunists and observers are included in the model.


In the model, the "Bank" is represented by two sections: the green one is the deposit area, while the red one is the withdrawal area. When partecipants enter the area with green patches, non-opportunist agents deposit two hours in the bank account related with their type of offered service. Instead, when they enter the area with red patches, both opporunists and non-opportunists withdraw two hours of the service they need, only if the relative bank account is not empty.
At time zero, every agent has monthly free time hours, a personal account of deposited and withdrawn hours, a personal budget, a constant type of offered service, a variable type of necessity. Opportunist condition is randomly assigned at a number of agents depending on the slider on the interface.
Agents are initially caracterized by the white color, meaning that their offered hours are equal to their withdrawn hours. When offered hours are major than withdrawn hours they become green, while when withdrawn hours are major than offered hours they become red.
At the beginning of the simulation there is a variable number of observers, whose color is black. They don't move until the observed satisfaction doesn't reach a certain level; then they gradually become clients of the "bank".
Clients satisfaction depends on: number of opportunist clients and number of time that they reach personal equilibrium.
Opportunists are members that want only to withdraw hours without any personal deposit. After withdrawing 4 hours, the "bank" prevents them from making any operation and they assume the cross shape and the color yellow.


Clicking the button "Setup" the simulation is ready to start. Then, with the button "Go", the agents start moving and interacting in the way explained above. To stop the simulation is necessary to press again the button "Go".

SETUP: button that allows to reset the model, prepares it to start and creates turtles and patches with initial setting values.

GO: button that makes the model start running.

GO ONE STEP: button that, if clicked, makes the model run only one cycle of the simulation.

NUMBER_OF_PEOPLE: slider that allows users to decide the number of agents in the model in a range from 5 to 500, with an increase of 1.

MONTHLY_FREE_TIME_HOURS : slider that caracterizes the agents. It provides the agents with a number of free time hours per month. The users can choose the amount in a range from 0 to 20.

MAX_NUMBER_OF_OPPORTUNISTS: slider that represents the maximum number of opportunists in the model. It is variable from 0 to 250.

NUMBER_OF_OBSERVERS: slider that the users can move to decide the number of observers included in the model, variable from 5 to 100.

DEBTORS / CREDITORS: monitors that count the number of clients debtors (red turtles) and creditors (green turtles).

OPPORTUNISTS_THROWN_OUT: monitor that counts the number of opportunists discovered ( yellow cross turtles).

SATISFACTION_OF_CLIENTS: plot that shows the trend of clients' satisfaction.


It is interesting to include some opportunists in the model. This condition has a negative impact on the satisfaction that increases more slowly. This situation affects the trust in the system and increases the time required for observers to enter the "bank".


The users can combine a low presence of opportunists and an high amount of free time hours to see a rapid increase of "bank's" clients. Also the initial number of agents affects the functioning of the model: if there are just a few members, global satisfaction grows very slowly.


Add more services with different "weight" in hours, depending on the qualification of the offered service.
Include in the model an estimation of the "bank's" costs relative to the ordinary administration.