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Building Empirically Plausible MAS 111involve decision weights for example and it must be possible to measure these.If, in fact, real agents do not make decisions using a BDI approach, they willhave no conception of weights and these will not be measurable or, worse,unstable artefacts of the measuring technique. Until they have been measured,these entities might be described as “theoretical” or “theory constructs”. Theyform a coherent part of a theory, but do not necessarily have any meaning inthe real world.Thus, despite some limitations and given the state of “normal science” insocial simulation, this chapter can be seen as a thought experiment. Could webuild MAS genuinely “based on” data? Do such MAS provide better understandingof social systems and, if so, why?3. The Case Study: Innovation DiffusionProbably the best way of illustrating these points is to choose a social processthat has not yet undergone MAS simulation. Rogers [18] provides an excellentreview of the scope and diversity of innovation diffusion research: the studyof processes by which practices spread through populations. Despite manyexcellent qualitative case studies, “normal science” in the field still consists ofstatistical curve fitting on retrospective aggregate data about the adoption of theinnovation.Now, by contrast, consider innovation diffusion from a MAS perspective.Consider the diffusion of electronic personal organisers (EPO). For each agent,we are interested in all message passing, actions and cognitive processing whichbears on EPO purchase and use. These include seeing an EPO in use or usingone publicly, hearing or speaking about its attributes (or evaluations of it),thinking privately about its relevance to existing practices (or pros and consrelative to other solutions), having it demonstrated (or demonstrating it). Inaddition, individuals may discover or recount unsatisfied “needs” which are(currently or subsequently) seen to match EPO attributes, they may actuallybuy an EPO or seek more information.A similar approach can be used when more “active” organisational roles areincorporated. Producers modify EPO attributes in the light of market researchand technical innovations. Advertisers present them in ways congruent withprevailing beliefs and fears: “inventing” uses, allaying fears and presentinginformation. Retailers make EPO widely visible, allowing people to try themand ask questions.This approach differs from the traditional one in two ways. Firstly, it isexplicit about relevant social processes. Statistical approaches recognise thatthe number of new adopters is a function of the number of existing adopters but“smooth over” the relations between different factors influencing adoption. It istrue that if all adopters are satisfied, this will lead to further adoptions through