Lanstein and Sharpe (LS) attempt to explain residual covariances between stocks on the basis of duration considerations. The results, by admission are mixed. Rather than to focus on these per se, I would like to further the work by making some suggestions with respect to the formal model development and the empirical tests-on the basis that both could be made crisper and thereby increase the value of what already is a contribution.

Ezequiel L. Machado, Willard T. Carleton, Financial Planning in a Regulated Environment, The Journal of Financial and Quantitative Analysis, Vol. 13, No. 4, Proceedings of Thirteenth Annual Conference of the Western Finance Association, June 20-26, 1978 (Nov., 1978), pp. 759-777

Richard C. Burgess, Bruce T. O'Dell, An Empirical Examination of Index Efficiency: Implications for Index Funds, The Journal of Financial and Quantitative Analysis, Vol. 13, No. 1 (Mar., 1978), pp. 93-100

Following Markowitz's [11] pioneering work on portfolio selection, it is customary to consider an individual's choice among several risky assets as a two-step procedure. First, given some general characteristics concerning his preferences, the decision maker chooses an efficient set of portfolios independent of his specific preference assessment. Secondly, an optimal portfolio is chosen from the efficient set given the individual's specific preferences.