David Blitzer on How Indices Work

David Blitzer

David Blitzer is a managing director and the chairman of the S&P Index Committee with overall responsibility for security selection for S&P’s indices and index analysis and management.  Dr. Blitzer is the author of Outpacing the Pros: Using Indices to Beat Wall Street’s Savviest Money Managers, (McGraw-Hill, 2001) and What’s the Economy Trying to Tell You? Everyone’s Guide to Understanding and Profiting from the Economy, (McGraw-Hill, 1997)

Dan Richards spoke with Blitzer at the CFA Annual Conference in May.

A video of this interview is available here.


Let's start by talking about how the S&P 500 is composed.

It is 500 companies.  We occasionally get phone calls, believe it or not, about how many stocks in the S&P 500.  It's overseen and maintained by a committee of nine people, all of whom work for Standard & Poor's and McGraw Hill, our parent company. 

We have a series of published guidelines and requirements for a company to be added to the Index.  It has to be a US company worth at least $3.5 billion dollars, trade with adequate liquidity, have at least half the stock available in public float.

We look at what sector or industry it belongs to, to keep the balance of sectors in the Index very close to the balance in the market.  And last, but certainly not least, it has to have made money.  It has to have four quarters of positive earning using generally accepted accounting principles (GAAP) net income. 

Over the years that has proven to be surprisingly important to the whole process.

What happens when someone reads that the S&P is up 10 points or down 10 points.  How is that calculated?

The Index is what we call a value-weighted index, or sometimes called market-cap or market capitalization-weighted index.  You take the market value of each company in the Index, and you add it all together, and you will get a number that is probably about $9 trillion right now. 

Well, $9 trillion is sort of a big number to report every 15 seconds.  In fact, by the time you get to the end of the number you've used your 15 seconds. 

We divide it by a scaling factor called the divisor, and that gives you the value of the Index, 1,142 or whatever it happens to be.  When you say the index is up, you take the value of all those companies – some go up, some go down at any given moment – the ones that went up outweigh the ones that went down, and after you scale it to a number that’s easy to handle, you come out with say 10 points, and maybe the Index has moved from 1,140 to 1,150.

You say that the companies are market weighted.  What would be the biggest company in the Index right now, the most important company today?

The biggest one is most likely Exxon Mobile which is a giant US or global oil company.

So you have a company like Exxon at the top of the list, and they are up 3% in a day, and then you have one at the bottom of the list not nearly as important, and they are down 3%.  So you've got two members, one’s up, one’s down, but they are not of equal importance to the index as a whole?

That's right.  Let's assume the other 498 stocks didn't move, just to make it simple.  Exxon mobile goes up 3%, and the number 500th company goes down 3%.  The Index overall will be up, because Exxon Mobil's weight in the Index is going to be much larger.  Its weight is probably 3, 3.5, or maybe 4%.  The one at the bottom we’re talking about is a very small fraction of a percentage point.