Non-smokers (potential smokers) (P(t))
The class of non-smokers (potential smokers) is increased by the recruitment of individuals into the nonsmoking class (at a rate uN). It is assumed that non-smokers can acquire smoking habits (and become smokers) via effective ‘‘contacts’’ with smokers (at a rate b = cq, where c is the average number of contacts per unit time; and q is the probability of becoming a smoker for a member of the P class following contact with a smoker). In other words, it is assumed that the acquisition of smoking habit is analogous to acquiring disease infection; the mode ‘‘contact’’ a potential smoker makes, the higher the likelihood of such an individual acquiring smoking habit. Although not considered in this study, other factors such as tobacco advertising and mass consumption of tobacco in the community can influence this process (and, hence, should be included in the function governing the generation of new smokers in the community). Finally, potential smokers suffer natural death (at a rate i), so that the rate of change of the population of potential smokers is given by
dP „r bPS
—— = uN uP. (1)
dt u N u w