Operational
Research Society: Simulation Study Group
UK
Simulation Society
Joint One Day
Meeting
Discrete Event
Simulation and System Dynamics:
Never the
Twain Shall Meet?
Wednesday 12th April 2000, 9:30am-4:00pm
Executive MBA Teaching Centre
Warwick Business School, University of Warwick, Coventry
When you want to build a simulation model how do you choose what kind of simulation approach to use? The choice between discrete event simulation and system dynamics may not be as straightforward as asking whether or not the real situation is discrete or continuous in nature. If you take a situation to be modelled, will a discrete event simulation expert automatically see the model as a discrete event simulation model and a system dynamics expert automatically see it as a system dynamics model? Or, alternatively, are there lessons to be learnt in how to choose which simulation method to use? The meeting will bring together presenters from both industry and academia to discuss their experiences using both discrete event simulation and system dynamics modelling. The day will finish with an informal discussion session where attendees will be encouraged to debate around issues that have arisen during the meeting.
09:30-10:00 Coffee
10:00-10:20 Introduction to the day
Susan Howick (Strathclyde University)
10:20-11:10 You Just Don’t Understand Me: Modes of
Failure and Success in the Discourse Between System Dynamics and Discrete Event
Simulation.
David Lane (London School of
Economics and Political Science)
This
paper examines two established simulation techniques; system dynamics (SD) and
discrete event simulation (DES). Brief familiarity with the two approaches
would lead one to expect there to be strong and mutually supportive
communication between the respective camps. In reality, the discourse between
SD and DES seems fraught with hostility, incomprehension and criticism. This
paper suggests that three modes of discourse may be determined.
Mode 1 concentrates
on the differences between the two and supports the conclusion that the present
institutional divide between them should continue and even be widened. Mode 2
concentrates on the similarities and proposes that they can be seen as aspects
of essentially the same idea.
It is argued that
these discourses are both failure modes because they have a superficial grasp
of the two approaches. This is the case with Mode 1 because it fails to
acknowledge the undoubted connection between them, from the broad level of
modelling aspirations down to some of the technical aspects of validation. Mode
2 also fails but because it incorrectly elides distinctive differences, for
example, concerning the decision problems that each approach is able to treat.
An account of the
differences and similarities leads to the proposal of Mode 3, in which a richer
understanding of SD and DES allows the two camps both to grasp opportunities
for making common cause in areas of similarity, and to have an informed and
respectful understanding of the reasons why there are areas of difference. Mode
3 is recommended as the only long term success mode for discourse between SD
and DES.
11:10-11:40 Coffee
11:40-12:30 Planes, Trains and Automobiles: A System
Dynamics Model of Large-Scale Military Operations with Continuous and Discrete
Elements
Steve Curram (HVR Consulting Services)
The presentation will look at a Powersim Constructor
system dynamics model of military operations for the Canadian Department of
National Defence that was developed to investigate the potential impact of the
Millennium Bug on their effectiveness.
The features of the problem situation meant that the choice of modelling
approach was not clear cut. The
presentation will review the pros and cons of each approach for the study and
highlight the structures that were used to implement discrete behaviour. It will conclude with lessons learned for
future modelling.
12:30-14:00 Lunch
14:00-14:45 The
Equivalence of Continuous and Discrete Simulation
Janet Lee, Pelham Barton and Andrew Tobias
(Birmingham University)
Given their distinct heritages, it is perhaps
unsurprising that continuous and discrete
simulation are treated somewhat separately in the literature and that
the majority of applications fall into just one of these categories rather than
two. The purpose of the investigation reported here was to compare the
structure and performance of a given continuous (system dynamics) model with
that of a supposedly equivalent discrete version.
To
compare the methods, the Staff Recruitment and Leaving (SRL) model of
Wolstenholme (1990) was chosen, for simplicity, as the starting point. The
model represents the movement of working staff in and out of employment in a
company where the recruitment rate is determined at any instant by the
shortfall in staff and the leaving rate is proportional to the size of the
workforce.
It
was implemented first in a simplified, system dynamics form, employing only
first order delays. Changes in staffing level, number of people in training and
also average leaving rate were then calculated by simulating from empty with
one set of parameters. Two equivalent models were then constructed - a discrete
event model and also a hybrid – so that profiles of the output measures from
the three models could be compared. Some surprising results emerged.
14:45-15:00 Tea
15:00-15:45 Open Forum Susan Howick (Strathclyde University)
15:45-16:00 Close
Booking a Place
To cover catering costs there is a £10 charge for this meeting. Please reserve a place by sending a cheque (payable to the UK Simulation Society), giving your name and contact details, to:
Dr Stewart Robinson, Warwick Business School, University of Warwick, Coventry, CV4 7AL
Directions to the meeting can be found at: http://www.wbs.warwick.ac.uk/maps/
The Executive MBA Teaching Centre is on the central campus between Radcliffe and Scarman House. (Note: since it is a new building it is not marked on the map.)
For further details please contact:
Stewart Robinson (tel: 01203-522132, email: stewart.robinson@warwick.ac.uk)
Simon
Taylor (tel: 01895 203389, email: simon.taylor@brunel.ac.uk)