besides serving as a potential basis for the oat, the market model,despite all its weaknesses,is also of interest on two grounds,first it produces estimates for the b's that play a central role in the capm note,however ,that estimating b's from past data alone is useful only to the extent that some degree of stationary in the relationship between asset returns and the returns and the return on the market is present. empirical observations suggest a fair amount of stationary is plausible at the level of portfolio , bus not of individual assets, on the other hand, estimating the b's does not require all the assumptions of the market model; in particular,a violation of the cov(e,ek) = 0, i =k hypothesis is not damaging.