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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/N<sup>2</sup>, 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/N<sup>2</sup> 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).
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/N<sup>2</sup>, 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/N<sup>2</sup> 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).
为了完整说明这个模型，箭头上标明了类别之间的传染率。在S和I之间，传染率假定为<math> d\left( S/N \right) /dt=-\beta SI/N^2 </math>，其中 <math> N </math> 是总人口，<math> β </math>是平均每人每次接触的数量乘以易感者和感病者之间接触传播疾病的概率，<math> SI/N < sup > 2 </sup > </math> 是易感个体和染病个体之间接触的比例，这种接触导致易感个体转化为染病个体。(这在数学上与化学中的'''<font color="#ff8000">质量作用定律the law of mass action</font>'''类似，在这个定律中，分子之间的随机碰撞导致化学反应，反应比例与两种反应物的浓度成正比)。
为了完整说明这个模型，箭头上标明了类别之间的传染率。在S和I之间，传染率假定为<math> d\left( S/N \right) /dt=-\beta SI/N^2 </math>，其中 <math> N </math> 是总人口，<math> β </math>是平均每人每次接触的数量乘以易感者和感病者之间接触传播疾病的概率，<math> SI/N^2 </math> 是易感个体和染病个体之间接触的比例，这种接触导致易感个体转化为染病个体。(这在数学上与化学中的'''<font color="#ff8000">质量作用定律the law of mass action</font>'''类似，在这个定律中，分子之间的随机碰撞导致化学反应，反应比例与两种反应物的浓度成正比)。