Proxies such as the MSCI measure only stocks that are traded and thus exclude most of the many asset classes that should be included in the index (bonds, derivatives, real estate, untraded assets and human capital). Clark, E. and Kassimatis, K., 2011. “An alternative measure of the ‘world market portfolio’: determinants, efficiency, and information content”, JOURNAL OF INTERNATIONAL MONEY AND FINANCE, Vol 30, n° 5, pp 724-748, Summarized in CFA Digest, 2011 provide a methodology for computing the "world market portfolio.