RT Journal Article SR Electronic T1 Asset Allocation JF The Journal of Fixed Income FD Institutional Investor Journals SP 7 OP 18 DO 10.3905/jfi.1991.408013 VO 1 IS 2 A1 Fischer Black A1 Robert B Litterman YR 1991 UL https://pm-research.com/content/1/2/7.abstract AB