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Publication - Research and analysis

Using Discrete Event Simulation to explore "what-if" waiting list scenarios in NHS Scotland

Published
30 June 2025
Directorate
Chief Operating Officer, NHS Scotland Directorate
Topic
Health and social care
ISBN
9781836917373

This publication explores the usability of the discrete event simulation method for modelling NHS Scotland planned care waiting lists, given the data available. As an initial case study the focus was on ophthalmology, and in particular cataract surgery.

View supporting documents Supporting documents

Modelling definitions

Conversion rate definition

The interplay between the two waiting lists, NOP and TTG, is not fully described via available data - partly because of limited data tracking patients moving between the waiting lists.

For this model it was therefore necessary to establish a placeholder variable to quantify the flow of patients between the two waiting lists. A crude approximation was derived based on the assumption that a certain proportion of people who attend an NOP consultation will be referred for treatment and therefore arrive on, or convert to, the TTG waiting list. This publication defines this proportion as "conversion rate”. In this report, conversion rate always refers to individuals converting from an NOP attendance to a TTG addition.

This is a simplistic average using published data that may not account for real world complexities, however this was considered acceptable for this piece of work which is focused on the utility of the modelling method.

The model considers two system modes which impact how the conversion rate is calculated.

Mode 1

The assumption is made that all patients that arrive on the TTG list are referred following an NOP attendance. Then the conversion rate is calculated as follows for any given month:

π‘π‘œπ‘›π‘£π‘’π‘Ÿπ‘ π‘–π‘œπ‘› π‘Ÿπ‘Žπ‘‘π‘’1= 𝑇𝑇𝐺 π‘Žπ‘‘π‘‘π‘–π‘‘π‘–π‘œπ‘›π‘  / 𝑁𝑂𝑃 π‘Žπ‘‘π‘‘π‘’π‘›π‘‘π‘Žπ‘›π‘π‘’π‘ 

There is a built in assumption that the referral to the TTG waiting list happens with no time delay.

Mode 2

This scenario takes into consideration that individuals can be referred to the TTG list from other sources in addition to NOP attendances e.g. directly after surgery on the first eye if they require surgery on both eyes. Then the conversion rate is calculated as follows for any given month:

π‘π‘œπ‘›π‘£π‘’π‘Ÿπ‘ π‘–π‘œπ‘› π‘Ÿπ‘Žπ‘‘π‘’2= (π‘‡π‘œπ‘‘π‘Žπ‘™ 𝑇𝑇𝐺 π‘Žπ‘‘π‘‘π‘–π‘‘π‘–π‘œπ‘›π‘  - 𝑇𝑇𝐺 π‘Žπ‘‘π‘‘π‘–π‘‘π‘–π‘œπ‘›π‘  π‘“π‘Ÿπ‘œπ‘š π‘œπ‘‘β„Žπ‘’π‘Ÿ π‘ π‘œπ‘’π‘Ÿπ‘π‘’π‘ ) / 𝑁𝑂𝑃 π‘Žπ‘‘π‘‘π‘’π‘›π‘‘π‘Žπ‘›π‘π‘’π‘ 

In this report, the rate at which TTG additions come from other sources is called the β€œdirect TTG rate”. β€œDirect” is used as these additions skip a new outpatient appointment.

Contact

Email: Emily.Henderson@gov.scot

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