The dollar variance of Synchrony returns is and finally the volatility is $162K.

I hope I bored you with this calculation, because it is boring. But this is a back-of-the-envelope calculation that helps for many tasks. For example: you can pull SYF and SP500 returns from a website or Bloomberg for the past year, estimate their vols, then perform a quick regression in Excel to estimate the beta of SYF to SP500 and then use the same calculation above to derive the idiosyncratic vol (do it!). Yes, there is a Bloomberg function for that (BETA GO), and yes, even Yahoo Finance reports the trailing 3-year beta. But suppose that you don't want a three-year beta. Or suppose that you don't want to include a certain date range in SYF returns, for example, the day of an idiosyncratic event that resulted in a one-off large return, and is not representative of the future volatility of the stock. There are many valid reasons for wanting to customize the estimation of vols and betas. Once you have understood the principle, you are the master of your own destiny, even if you pull the data from a commercial model.

Figure 3.3 The variance of the sum of two independent random variables is equal to the sum to the variances of the two random variables. You can interpret the standard deviations as sides of a right triangle.

Now that we have understood the risk decomposition of a single stock into market and idio, let's extend to a whole portfolio. The parameters from the risk model and the portfolio are in Table 3.5.

Table 3.5 Synchrony, Wal-Mart and SP500 risk parameters, together with holdings for each asset.

Field	SYF	WMT	SPY
Beta	1.2	0.7	1
Daily Market Vol (%)			1.4
Daily Idio Vol (%)	1.2	0.5	0.0
Net Market Value	$10M	$5M	$10M

The daily PnL (“Profit and Loss”) of the portfolio is defined as the sum of the holdings times their respective returns:

      Now, we can use Equation (3.1) to replace returns with their components, and rearrange the terms:

      (3.2)

      The performance of the portfolio can be split into the contribution of two terms: a market term and an idiosyncratic one. This is a simple example of performance attribution.

      The beta of the portfolio is . The important thing to notice is that the beta of the portfolio is the sum of the betas of the individual holdings. This overall portfolio beta is expressed in dollars, and usually is called the dollar beta of the portfolio. This dollar beta, multiplied by the market return ,

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