variance in firm size changes, o2. or decrease less the greaterthe o-o, will either increase mo is, provided that 2 po. Weiss goes on to argue that there is greater dispersion in firm size changes in industries of durables and semi-durables where style and model change is the prevalent form of competition: the existence of a stock of previously produced goods might well result in new automobile models or new edition of introductory texts, even in the presence of perfect price collusion Increases in concentration are thus anticipated to b positively correlated with frequent style of model change -it will be highest in industries with differentiated durable and semi-durable goods Birth and Death Process (simon and Bonini, 1958) Allow births into the lowest size class of the distribution. they allow Gibrat's law to operate therefore above some mes of firm. Firms are assumed born into this smallest size class at a constant rate e ( the probability of entry). For firm sizes 'sufficiently above mes the distribution is approximately Pareto with an inequality parameter a where a1-6 where 0 =g/G, G-net growth of assets of all firms in an industry in the period, g= part of G due to new firms. Serial Correlation Models (Ijiri and simon) In these models growth is serially correlated E(g,1)=k(t)zlg.B 8t is the rate of growth during the t time interval E( ) is the expected value operator k(t)is a function of time, the same for all firms B is the fraction that determines how rapidly the influence of past growth drops out 4