Beta Essay

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The concept of beta has gained prominence due to the pioneering works of Sharpe (1963), Lintner (1965) and Mossin (1966). There are many studies that examine the behaviour and nature of beta. These studies include the impact of the length of the estimation interval, the stability of individual security beta as compared to portfolio beta, factors influencing the beta as well as the stability of beta in various market conditions.

In theory, market capitalisation weighted indices are preferred as compared to equally weighted indices due to the fact that they are superior proxies and are consistent with the true market portfolio. Some practitioners argue that there is a perceived segmentation between the Resources, Financial and Industrial sectors on the JSE and consequently prefer to use the Financial and Industrial Index as an overall market proxy for stocks belonging to this category. Choosing the correct market index in order to regress against, is a vital aspect. Stambaugh (1982) identifies that the CAPM tests are generally insensitive to the choice of market proxy. However, many believe that the broader the selected indices, the better the market proxies. In the UK market, the two main indices used are the FT 100 which is made up of the top 100 companies and the FT All Share Index which is made up of all companies traded on the exchange.

The length of the estimation period is also significant in the estimation of beta. Blume (1971) depicts empirically that the stability of individual beta increases as the time of the estimation period increases. Similar results were obtained by Altman (1974) and Baesel (1974) who with the use of monthly data, estimated beta for estimation periods of one year, four years, six years and even ni...

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...phases. Fabozzi and Francis (1977) conducted a study testing the differential effect of bull and bear market conditions for 700 individual securities listed on the NYSE. It was found that the estimated betas of most of the securities were stable in both market conditions. However, Ray (2010) conducted a similar study over a period of ten years using monthly returns of 30 stocks. The results obtained were both mixed and inconclusive. Bowie and Bradford (1997) found that the tests of beta stability are difficult to interpret on their own. Gombola and Kahl (1990) suggest that an OLS estimate of beta requires an estimation period during which the relationship between the market return and the stock return remain stable. However, without this stability, an alternative for forecasting a time-varying relationship such as the Bayesian adjustment process will be required.

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