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

Introduction and key points

This publication explores the utility of Discrete Event Simulation as a modelling method to test what-if scenario impacts on NHS Scotland waiting lists. Discrete event simulation was used to replicate planned care waiting lists across outpatients, inpatients and day cases.

The aim was to test the usability of the simulation method for modelling NHS Scotland planned care waiting lists, including ascertaining if suitable data were available to support the simulation method.

As an initial case study the focus was on ophthalmology, and in particular cataract surgery. In the rest of the report this will be referred to as the “cataract case study”. This method could be applied to other areas of planned care, and expanded to cover ophthalmology more comprehensively if the data were available.

The model was used to look at the impacts of 1) a process change i.e. a change in how patients can flow through the system, and 2) a productivity change due to increased cataract surgery throughput .

Two discrete event simulation modes were developed, with and without direct TTG additions, that can reproduce baseline TTG list size using a process map that was developed alongside published PHS data to build distributions for the variables. The simulation mode with direct TTG additions allows patients requiring surgery on a second eye to be directly added to the TTG list following surgery on the first eye. This is not allowed in the mode without direct TTG additions where instead it is assumed that after surgery on the first eye, patients finish their journey and have to start again from the beginning for the second eye.

The two simulation modes were compared in a scenario where cataract surgery throughput was increased significantly. The mode without direct TTG additions projected that the average TTG list size would fall to less than 5,000. The mode with direct TTG additions projected that the average TTG list size would only decrease moderately to just below 20,000. Direct TTG additions in this simulation have a dampening effect on the potential productivity gains due to increased cataract surgery throughput.

The modelling process provides additional value by facilitating an improved understanding of the system. Small details can have big impacts as shown in this case study where cataract patients can be added directly back onto the TTG list for surgery on their second eye – depending on how often this occurs in practice could influence how successful any productivity improvements are on TTG activity.