Model fidelity describes the extent to which a model can replicate reality. Fidelity depends on the purpose of the model. Some models, such as an aircraft flight simulator need to strongly replicate reality to fulfil their purpose of training people, thus they have a very high fidelity, the analogue. However other models may only need to illustrate a principle, such as the motion of a train. It could be made analogue for a specific train, but people often learn more from a simpler illustration.
Model fidelity also depends on the ability of the mathematics to handle complexity. A more illustrative model may only be possible, such as the interaction of many species, as the governing laws may not be fully know, or data is not detailed enough to calibrate fully.
Further details in Morecroft J. (2007).
Types of Models
Every mathematical model is an attempt to replicate aspects of reality. Mathematics is the language used to express that model. Thus there are many types of models depending on the ability of the mathematics to handle the complexity of the real world and the purpose of the models. Complexity and purpose help determine the type of model produced.
Models can be classified according to their:
- Fidelity, the extent to which models are intended to replicate reality;
- Theoretical Support, a logical framework of consistent laws which enable models to explain a wide range of behaviour;
- Methodology, a logical collection of methods and principles, mathematical and computational in this case, that enable a model, and any associated theory, to be expressed and analysed.
In summary: "One or more methodologies are used to express a theory of a certain strength which in turn supports a model of a certain fidelity."
A model can be described as "successful" by the degree to which it replicates reality and usefully fulfils its purpose. A model does not need to use the richest methodology, have the strongest theoretical support or the highest fidelity to be useful. For example a simple back-of-the-envelope calculation that improves one process, in one industry, at a given time is highly useful and thus successful. Even if a model is not "rocket science" it can still instil confidence.
|Analogue||This is the highest fidelity. The model has a high degree of detail and scaling. Parameter values can be determined numerically with realistic units. Algebraic and functional relationships are closely constrained. Model variables can match measured data. Almost a point by point comparison with the real world situation.|
|Illustrative||This is a mid-range fidelity. Parameter values and variables can be given plausible values. Although numerical accuracy is less important than the analogue case, the theoretical mechanisms to explain growth, such as feedback loops, transition laws and rate changes, are realistic.|
|Metaphorical||This is the lowest fidelity. Here a general principle is taken from one domain and applied to another, without too much regard to numerical values or scale. The important issue is the type of structure, such as feedback loops, and the corresponding types of behaviour, such as the shape of a graph over time.|
The Limited Enthusiasm model is towards the high fidelity end of the spectrum as all its variables and parameters have well defined meaning and measurable values, more analogue than illustrative.
With demographics added the model becomes a little less analogue as some of the parameters are harder to estimate. Nevertheless they are in principle measurable. The absence of cohorts makes the birth and death processes a little more illustrative than analogue. The model provides principles and can be calibrated for congregations and denominations and fits within the standard genre of population modelling.
The Spiritual Life model is illustrative. Realistic values can be given for the population numbers, and compared with data, but Spiritual Life, which is a soft variable, is in principle hard to quantify. However the model can tell plausible stories with the level of Spiritual Life being compared with proxy variables. The model provides principles, but would not be easy to calibrate for a particular church grouping.
The Limits to Growth models are metaphorical. Although they could be made illustrative, and perhaps even analogue, that is not their purpose. They are constructed to show how growth limits from other sciences, such as the environment and business, are also significant for understanding barriers to church growth.
Not all models can be elevated to the lofty position of a "theory". However all models have a degree of theoretical support. For some situations a stronger theoretical support is essential in order to build confidence. This is particularly true for models which are hard to calibrate, or have soft variables, ones that are hard to measure. However for models of specific situations, where a particular problem needs to be solved, and/or where there is a wealth of data to calibrate the model, then a strong theoretical support is not always necessary. Confidence is built from the model's ability to solve the problem.
What follows is my own rough outline of the degree of theoretical support for a model, no doubt incomplete!
|Universal||The theory has been tested in a wide variety of situations, is basic enough that it pervades many domains and is the basis for many other theories and models. The theory is "atomic" describing simple principles from which more complex situations are constructed. The theory is universally accepted such that there are no realistic alternatives, and has been accepted for a long period of time.
Newtonian mechanics is the obvious example. A universal theory would be unlikely in a subject as complex as social modelling, which depends on the highly variable, and sometimes independent, behaviour of actors.
|Strong||A strong theory is one that has been tested in a wide variety of situations and is the basis for other theories and models. It is atomic, describing simple principles. It has a long track record as the accepted theory, although alternatives are possible.
Einstein's General Relativity is a good example. It is the accepted theory of gravitation, although there are alternatives. It is well tested in a variety of situations, but there are some observed anomalies in astrophysics for which a modification of the theory might be needed as an alternative to the dark matter and dark energy hypotheses.
|General||Although the underlying theory for the model may be only one of many, the principles it expresses could be applied across a wide variety of domains and scenarios. Many metaphorical models would be in this class. However the theory is often only partial, describing universal phenomena but in a restricted environment.
The negative exponential law is one such example, used for cooling, radioactive decay and deaths in populations. The limits to growth metaphor is another, describing, for example , species, human populations and market penetration of companies.
|Phenomenological||Similar to the above, but without the cross-domain generality. Capable of being repeated for other examples of the same phenomena in the same domain. The People's Express model (Morecroft, 2007) is an example. Although a model of a specific company (People's Express Airlines) was in mind, the model was used to illustrate the consequences of under investment and could thus be applied to a range of companies. However the model structure has less relevance outside the business world.|
|Specific||Similar to above but the model is only for one example in a given domain. An example would be a model of a specific company without concern as whether this may represent a class of similar companies.|
|Common Sense||A set of individual hypotheses that make sense, but do not constitute a cohesive set of rules.|
The Limited Enthusiasm model is part of the general theory of word-of-mouth models, which are in turn based on models of the spread of disease. These are based on the mass action principles that expresses how two differing sub populations interact. Other examples of models based on this theory are in the social diffusion references and the epidemic modelling references. The model with demographics added has the same theory.
The Discipleship model is phenomenological. It is a model of a generic church congregation. It could be applied to most congregations (so not "specific"), but has no applicability outside a church context (thus not "general"). It uses various familiar model constructs.
A methodology is a collection of rules and principles that allow a model to be constructed, expressed and analysed. The rules must make logical sense and thus be expressed in terms of mathematical and computational constructs. Mathematics contains a range of constructs from calculus to statistics, probability to symbolic logic. To be a modelling methodology rules need to be added that relate the mathematics to the real world.
The following are three of many dynamical modelling methodolgies. They are dynamical as the type of modelling concerns behaviour over time.
To date all the church growth models presented on this site are either differential equation models, or system dynamics. However there are also agent based versions of some of these models.