For the full specification of the model, the arrows should be labeled with the transition rates between compartments. Between "S" and "I", the transition rate is assumed to be "d(S/N)/dt = -βSI/N2", where "N" is the total population, β is the average number of contacts per person per time, multiplied by the probability of disease transmission in a contact between a susceptible and an infectious subject, and "SI/N2" is the fraction of those contacts between an infectious and susceptible individual which result in the susceptible person becoming infected. (This is mathematically similar to the law of mass action in chemistry in which random collisions between molecules result in a chemical reaction and the fractional rate is proportional to the concentration of the two reactants).