The Russell Indexes such as 1000 (large companies), 2000 (small companies), and 3000 (total market) are calculated based on total market capitalization (number of outstanding shares times the share price).
When people talk about indexes, they usually assume a statistical framework captures companies for the index. However, the S&P 500 actually has a committee that adds/subtracts companies to be included.
For instance, last year Advanced Micro Devices, Raymond James, Inc., and Alexandria Real Estate Equities were added. Deleted were Urban Outfitters, Frontier Communications, and First Solar.
The S&P 500 is calculated based on the weighted average market calculation: The sum of the adjusted market cap divided by an index divisor which is proprietary to Standard & Poors resulting in the weighting skewed to larger companies.
The same is true for the Dow Jones Industrial Average (DJIA). Over the last several years, a committee made the following changes:
|General Motors||Cisco Systems||2009|
|Bank of America||Goldman Sachs||2013|
Also, the calculation for the DJIA is based on price weighted as opposed to market weighted. The sum of the stock prices is divided by what is called the Dow Divisor created by Dow Jones.