Question relating to queues / delays / waiting

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PaulMcg
Junior Member
Posts: 2
Joined: Tue Mar 17, 2020 3:15 pm
Vensim version: PLE

Question relating to queues / delays / waiting

Post by PaulMcg »

Hi, I am new to Vensim / SD (have currently have access to PLE / PLE+), I have looked through the manual/tutorials, however many of the functions are not available in PLE+ so I am unable to become familiar with the functionality. Is the following possible in Vensim PLE+ or Pro, Ventity (trial version).
The particular element of the modelling I am struggling with is around capacity and duration of stay with results in a backlog / waiting.
we have a number of units/people entering a system each day and a fixed capacity/number of beds. However, I would like to extend the duration (days) each set of units/person occupies a bed.
For example:
Day 1) 2 people (group a) enter the system and occupy the 2 available beds, They will remain the the beds for 2 days.
Day 2) 3 People (group b) enter the system and are unable to move into a bed (occupied by group a) so have to wait.
Day 3) 2 People arrive (group c). group a exit the system and 2 members of (group b + c combined) move into the 2 available beds and the 3 remaining members of group b + c wait.
The process continues.

I have been going over this for days now, I am able to model the flow and capacity and I have tried a number of functions/methods including Step, IfThenElse to reduce capacity to zero when the bed is occupied and Delay Fixed however when using delayed fixed it moves the whole group (when output limited by capacity) and not delaying each group entering the beds by the by the number of selected days. So far I have been unable to find a solution and would really appreciate any input.
thank you
Paul
tomfid
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Joined: Wed May 24, 2006 4:54 am

Re: Question relating to queues / delays / waiting

Post by tomfid »

This is doable with stocks and flows as well as with the fancier queue and delay functions. A lot depends on whether you need the detail of integer people or not (i.e. are you building a discrete event simulation, or a continuous SD model). Do discrete beds and people matter, or is this more strategic?
PaulMcg
Junior Member
Posts: 2
Joined: Tue Mar 17, 2020 3:15 pm
Vensim version: PLE

Re: Question relating to queues / delays / waiting

Post by PaulMcg »

Hi tomfid, thank you for your response. I currently work in local government (adult/children social care, homelessness etc.) and there is a great deal of focus around the flow of people from care settings into hospital and back (bed and staff capacity) I generally use Excel/PowerBI, R, and SQL I feel (with my very limited understanding) a SD/stock and flow approach could be very beneficial. In response to your question, Initially, I am looking to model backlog/waiting, admissions/bed occupancy and discharges units: person and person/day. I don't need to track individuals just the daily and cumulative counts. If I am successful, I would look to undertake a similar process modelling and combining other elements of the social care system to help understand how the system functions (pathways, bottlenecks, limited / over capacity etc.). I have attached my first attempt at modelling a basic system (I have replaced the two data sources with lookups).
Thanks Paul
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Administrator
Super Administrator
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Joined: Wed Mar 05, 2003 3:10 am

Re: Question relating to queues / delays / waiting

Post by Administrator »

I'd definitely start by fixing the unit errors. These really help point out errors in model structure and assumptions.

Other than that, the model looks like it's a good start.
Advice to posters seeking help (it really helps us to help you)
http://www.ventanasystems.co.uk/forum/v ... f=2&t=4391

Units are important!
http://www.bbc.co.uk/news/magazine-27509559
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