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Concept
Market portfolio
Summary
Market portfolio is a portfolio consisting of a weighted sum of every asset in the market, with weights in the proportions that they exist in the market, with the necessary assumption that these assets are infinitely divisible.
Richard Roll's critique states that this is only a theoretical concept, as to create a market portfolio for investment purposes in practice would necessarily include every single possible available asset, including real estate, precious metals, stamp collections, jewelry, and anything with any worth, as the theoretical market being referred to would be the world market.
There is some question of whether what is used for the market portfolio really matters. Some authors say that it does not make a big difference; you can use any representative index and get similar results. Roll gave an example where different indexes produce much different results, and that by choosing the index you can get any ranking you want. Brown and Brown (1987) examine this, using different indexes such as stocks only, stocks and bonds, and stocks plus bonds plus real estate. They find that using a market that includes real estate produces much different results. For example, with one measurement most mutual funds have alpha close to zero, while with another measurement most of them have significantly negative alpha.
Most index providers give indices for different components such as stocks only, bonds only, et cetera. As a result, proxies for the market (such as the FTSE 100 in the UK, DAX in Germany or the S&P 500 in the US) are used in practice by investors. Roll's critique states that these proxies cannot provide an accurate representation of the entire market.
The concept of a market portfolio plays an important role in many financial theories and models, including the capital asset pricing model where it is the only fund in which investors need to invest, to be supplemented only by a risk-free asset, depending upon each investor's attitude towards risk.
Sharpe (2010) notes that many investors are at least targeted to a fixed ratio (e.
Official source
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