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compromise and explicit discussion about these issues is fundamental to constructing a good model . Policy analysts also need to understand the model development process in order to be involved at the key points and guide the modeling to achieve the policy objectives .
I - Background : What Makes a Good Model ?
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use the term ‘ model ’ to mean an explicit representation of relevant features and relationships of some target system ( adapted from discussion in Greenberger et al ., 1976 , pp . 48-49 ). This definition has three elements :
• Explicit representation : the model is constructed using a language , diagram , or some other communication medium that is independent of the modeler ;
• Relevant features and relationships : the model is simpler than the full target system and there is some purpose or question that guides the decisions about what to include and exclude in the representation ; and
• Target system : the model is ‘ of ’ something .
For a policy model , the target system is the issue of interest . As an ‘ issue ’ is intangible and difficult to represent , the target system is roughly ‘ all the things we need to know about to understand the issue ’. It potentially includes the people and groups affected by the issue , their beliefs and actions , applicable laws and procedures , resources and any other entity or factor involved in creating the issue or affected by it . The target system is not restricted to the issue as it exists , but can also include the factors and relationships relevant to hypothetical descriptions about the future or potential policy options .
The second part of the definition means that all models are abstractions ; some characteristics of the target system are excluded completely from the model or included in simplified form . This is because any model that fully replicated the target system would be just as difficult to comprehend as the full system . For example , a road map is useful because it can display how to get from one place to another in a single view . A map that included every feature of an area would need to be the same size as the area being mapped .
What is relevant , and therefore included in the abstraction , depends on why a model is required . Continuing the example , a map for travelling between cities is at a different scale from a map to find particular locations within a city . Thus , models of the same target system may be very different depending on their intended use . Much of the craft of modeling is in deciding which relationships to include at what level of detail . This involves selecting , and often compromising on , what is most relevant .
The key feature of an ‘ explicit representation ’ is that the portrayal is independent of the modeler and expressed in some common language . Only by communicating their different understandings of a situation can people investigate the similarities and differences between their perspectives on what is relevant .
There are many different types of expression or representation languages used for models . In general , policy models use mathematical equations , computer code , or diagrams . Less frequently , models can be in
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M . Porter , ������������������������������������������������������������������������� ( New York : Free Press , 1980 ).
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