Computing Beta

I will try to use as little math and jargon as possible…

Beta is one way to look at a stock’s behavior relative to the rest of the stock market.  The most common way to compute beta for a stock is to compare its price over a 3 year period versus the S&P 500.  The beta for a stock can be computed with daily, week, monthly or other data.  Generally the difference in the final answer is small between these.  Personally, I favor a beta based on daily closing values, but for this blog post I’ll stick with a monthly beta computation.

Lets compute beta for CSX versus SPY (an S&P500-based index EFT) using Microsoft Excel 2010:

  1. Go to Yahoo Finance and type in ticker symbol CSX.
  2. Click on “Historic Prices” and set the range from Oct 18, 2007 to Oct 18, 2010.  Select the “Monthly” radio button.
  3. Scroll to the bottom of the page and click “Download to Spreadsheet.”
  4. When prompted select “Open With -> Microsoft Excel”.
  5. Cut and paste the data in the “Adj Close” column to a new spreadsheet.
  6. Repeat the above process for SPY.  Put the SPY data in a column adjacent to the adjusted CSX closing price data.
  7. Compute the variance of SPY for example SPY data points e.g. “=VAR.S(C4:C40)”
  8. Compute the covariance of CSX with respect to SPY e.g. “=COVARIANCE.S(B4:B40,C4:C40)”
  9. Beta is, by definition, the value in step 8 divided by the value in step 7.  However I have found this not the case when using the MS Excel 2010 formulas above.  The next steps tell how I “fix” this beta.
  10. Compute average values for CSX and SPY: e.g “=AVERAGE(B4:B40)” and “=AVERAGE(C4:C40)”
  11. The fixed value is the result of step 9 *  the SPY average/the CSX average.  This is CSX’s 3-year, monthly beta.

Using this method, I compute a beta for CSX of 1.00.  This is a fair bit different that the value of 1.24 reported by Yahoo Finance.  I used the same process for MSFT and compute a beta of 0.93 versus Yahoo Finance’s 1.03 for Microsoft stock.  Looking for a more out-there beta, I repeated the process for C (Citigroup). I computed a beta of 4.39 versus Yahoo Finance’s 2.65.   For another comparison Google Finance reports betas for CSX, MSFT, C of 1.2, 1.05, and 2.54.  Finally, MSN Money reports betas of 1.21 ,1.06, and 2.55.

It irks me that 1) these 3 finance sites don’t detail their beta-computation methods, 2) They produce different results, 3) My method produces different results, 4) MS Excel doesn’t [I don’t believe] offer a beta or function, 5) I have to tweak MS Excel data to get a more reasonable beta computation.

Be that as it may, I managed to explain one way of computing beta, and did so with a minimum of math.  Please feel free to flame this post and tell me a better way.  Until then feel free to try my method, or create your own modified method.

P.S. — I did some web searching and found an alternate method that is pretty decent:

They also perform a monthly 3-year beta computation. I like that it is clear, correct, and easy to follow.   I don’t like that it uses an older version of Excel and that it requires graphing and essentially reading the numbers off of the graph.

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