The discussion of computing statistical values for portfolios containing missing data elements derives from the following references:
 Little, Roderick J.A. and Donald B. Rubin. Statistical Analysis with Missing Data. 2nd Edition. John Wiley & Sons, Inc., 2002.
 Meng, Xiao-Li, and Donald B. Rubin. "Maximum Likelihood Estimation via the ECM Algorithm." Biometrika. Vol. 80, No. 2, 1993, pp. 267–278.
 Sexton, Joe and Anders Rygh Swensen. "ECM Algorithms That Converge at the Rate of EM." Biometrika. Vol. 87, No. 3, 2000, pp. 651–662.
 Dempster, A.P., N.M. Laird, and Donald B. Rubin. "Maximum Likelihood from Incomplete Data via the EM Algorithm." Journal of the Royal Statistical Society. Series B, Vol. 39, No. 1, 1977, pp. 1–37.