Single Index Stock Market Model Using Single Regression Line. Essentials of Investments. CFA Exam.

IN this video, I discuss the single index stock market model using regression line.
The single-index stock model (SIM) is a simple asset pricing model to measure both the risk and the return of a stock.
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To simplify analysis, the single-index model assumes that there is only 1 macroeconomic factor that causes the systematic risk affecting all stock returns and this factor can be represented by the rate of return on a market index, such as the S&P 500.

According to this model, the return of any stock can be decomposed into the expected excess return of the individual stock due to firm-specific factors, commonly denoted by its alpha coefficient (α), the return due to macroeconomic events that affect the market, and the unexpected microeconomic events that affect only the firm.

The term {\displaystyle \beta _{i}(r_{m}-r_{f})}\beta_i(r_m-r_f) represents the movement of the market modified by the stock's beta, while {\displaystyle \epsilon _{i}}\epsilon_{i} represents the unsystematic risk of the security due to firm-specific factors. Macroeconomic events, such as changes in interest rates or the cost of labor, causes the systematic risk that affects the returns of all stocks, and the firm-specific events are the unexpected microeconomic events that affect the returns of specific firms, such as the death of key people or the lowering of the firm's credit rating, that would affect the firm, but would have a negligible effect on the economy. In a portfolio, the unsystematic risk due to firm-specific factors can be reduced to zero by diversification.

The index model is based on the following:

Most stocks have a positive covariance because they all respond similarly to macroeconomic factors.
However, some firms are more sensitive to these factors than others, and this firm-specific variance is typically denoted by its beta (β), which measures its variance compared to the market for one or more economic factors.
Covariances among securities result from differing responses to macroeconomic factors. Hence, the covariance of each stock can be found by multiplying their betas and the market variance:
The single-index model assumes that once the market return is subtracted out the remaining returns are uncorrelated: