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
10:00-10:20Introduction to the day
Susan Howick (Strathclyde University)
10:20-11:10You 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
11:40-12:30Planes, 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.
12:30-14:00Lunch
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
15:00-15:45Open 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:
{ HYPERLINK http://www.wbs.warwick.ac.uk/maps/ }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:
[log in to unmask])
Simon Taylor (tel: 01895 203389, email:
[log in to unmask])
Dr Stewart Robinson
Lecturer in Operational Research and Systems
Warwick Business School
Warwick University
Coventry
CV4 7AL
UNITED KINGDOM
Tel: 44-(0)1203-522132
Fax: 44-(0)1203-524539
Email: [log in to unmask]
Home Page: http://users.wbs.warwick.ac.uk/ors/slrhome.htm
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
