Previous studiesIn order to test the usefulness of real estate as a portfolio diversifier, price indexes are needed to compute holding period returns. Whereas price indexes are readily available for financial assets, this is not the case for real estate owing to the lack of transactions, the scarcity of data and the heterogeneity of assets. Several methods are used to construct price indexes for real estate. These include appraisal-based series, indexes constructed with securitized data, indices of the average of transactions prices, hedonic models and the repeat sales method. Although the first two methods have serious limitations, they have been widely used in empirical studies as they are easier to use and less data intensive. Very few studies have used hedonic indexes in order to test the attractiveness of real estate as an asset class. Indexes constructed from averages of transactions prices are usually discarded as the biases are too serious. To date, no study has made use of data derived from the repeat sales method to investigate the opportunity of including real estate in mixed-asset portfolios. Thus almost all studies have used either appraisal-based indexes or securitized data, while few have relied on the hedonic approach.To construct efficient portfolios, it is necessary to have information on the average return and risk of each asset class, as well as on the correlation coefficients between each pair of assets. As could be expected, the results vary quite substantially depending on what data series is used for real estate. When appraisal-based indexes are used, very low risk levels are found for real estate. Ibbotson and Siegel (1984), for instance, report a standard deviation of 3.7 per cent for US real estate for the period 1947-1982, compared with a standard deviation of 17.5 per cent for stocks and 9.7 per cent for bonds. Moreover, correlations between real estate returns and the returns on financial assets are usually found to be negative. Hartzell et al. (1986) report a correlation of -0.12 between real estate and stocks and a correlation of -0.39 between real estate and bonds. When such parameters are used to construct efficient portfolios, real estate is heavily represented in mixed-asset combinations. Webb and Rubens (1987), for instance, find that real estate should have constituted 83 per cent of a US portfolio over the period 1947-1959, 98 per cent over the period 1960-1972 and 91 per cent over the period 1973-1984. Similar results have been reported for the UK. MacGregor and Nanthakumaran (1992), for instance, find that the optimal weight for real estate goes up to 88 per cent in low return and low risk portfolios for the period 1977-90.These results, however, are not convincing as several authors have documented the appraisal smoothing which leads to downwardly biased standard deviations (Geltner, 1993;Geltner et al., 1994). When securitized data are used for the USA, the return and risk parameters of real estate are similar to those of common stocks (see Ross and Zisler, 1991). Moreov...