Grinold's fundamental law says that the expected information ratio of an active investment strategy, IR, is given by the information coefficient (IC) multiplied by the square root of the breadth, BR, which is a measure of diversification. In words, as Grinold says, "High information ratios follow from high levels of skill and/or high breadth or diversification." The presumption is that information ratios are the primary measure of value added. In a subsequent development the transfer coefficient, TC, was incorporated as a multiplier such that the extended fundamental law is given by IR = IC x sqrt(BR) x TC. The transfer coefficient, TC, is a measure of efficiency: to what extent do transaction costs and/or other constraints cause the actual portfolio to vary from the optimal portfolio?

Which of the following is the BEST measure of breadth, BR, in the fundamental law?