Yes, this medical article is based on the results of a mathematical model based on various variables. It's kind of like the projections you see on the news these days about the estimated path of a hurricane, except in this case, the model is looking backwards in time, to estimate the probability that the avian flu was passed human-to-human versus gettting infected some other way. The hurricane mathmetical model has variables like ocean temperature, air temperature, direction of prevailing winds, etc. This avian flu model has things like duration of the overall outbreak, time between infections, the size of the households, how often their paths cross, demographic info, etc.