A third compromise involves choice of the market index. Theory dictates that RRR is the return on the market portfolio, an unobservableportfolio consisting of all risky assets, including human capital and other nontraded assets, in proportion to their importance in world wealth. Beta providers use a variety of stock market indices as proxies for the market portfolio on the argument that stock markets trade claims on a sufficiently wide array of assets to be adequate surrogates for the unobservable market portfolio.