Theory Behind the CYPMH Referral System Management Tool

The CREST is based on the principles of queueing theory.  Queueing theory / queue analysis is the mathematical study of waiting lines, or queues. It uses a measure called 'erlang' to describe utilisation of an available capacity in a queue. They are named after a Danish mathematician who invented the fields of traffic engineering and queuing theory in the early 1900’s. 

Erlang is actually a 'family' of equations that are useful for different aspects of queue planning and you may see examples of Erlang equations and approaches more often applied or described in relation to Network Traffic, call handling in call centres and telephone exchanges. The equations can also be applied across many aspects of healthcare planning as well. 

Think of Erlang as a translator box, where you put in values like calls per hour, handle times, and number of agents available. These are “deterministic” values, in that they are easily measured. The outputs from the translator box provides answers to “probabilistic” questions such as, “What are the odds a call will have to wait in a queue?” or “What are the odds a call will wait more than 30 seconds?”. This is the 'engine' of the CREST.


When applying Erlang calculations to healthcare settings, we are translating 'calls per hour', 'handling times' and 'number of Agents' to 'referrals per week', 'points of contact' and 'number of appointments/beds available'.

Erlang for CReST