A Vensim based model
We know stock and flow are determinant to represent a system.
Delay could be used to simulate a process without change during
it. But sometime inflow or outflow adds in the process and
changed the flow of elements. Delay function could not work
successfully to model the real situation.
We can think about the hiring of new employees, the
demography study of population aging, the aging of equipment,
the supply. Delay is not allowed representation. We need aging
chain structure represented in the picture.
Stock is called cohort. In each cohort individual is mixed. The
entry and exit probability of each individual is independent.
Number of cohort is the result of required approximation within
each cohort. We can also use one year cohort and individual
enters and moves out of the level each year. The transition rate
cohort is generally a first-order delay. Sometime pipeline delay is
used to represent the behaviour.
Demographic study uses aging structure. Health system and
disease simulation could be improved if aging chain will be used.
Biological system (such as insect development) and social
system (such as marriage duration) could be easy captured by
aging chains. In general an aging chain structure is used to
represent age-dependent system (Sterman, 2000).
Eberlein and Thompson (2013) discuss a new way to represent
an aging chain. The function mathematic is described by
Eberlein and Thompson (2013) and Eberlein and coll. (2012).
This new way is called continuous cohorting and means “the
member of each age cohort are update on every computational
interval so that there is no anomalous up and down within a
year and no need for an extra indirectly measured cohort to help
with initialization”. The function creates an age disaggregation
of population so continuous complete movement of population
between two cohorts at each strep is possible. The discrete cohort
shifting is overcame and the tracking of individual are more
Continuous cohorting could be applied to any (animate and
inanimate) population allowed the best approximation to
behaviour. Continuous cohorting rather than aging chain is
recommended when we are looking at demographic or aging
aspects of the real world.
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