This is an interesting question, at least to me. Stock Indices are generally weighted by the Market Capitalization, which is the Total Value of Equity (Stock) Shares. So, let’s assume there are two corporations in an index, one with a total market value of $100 Billion and the other $200 Billion. In a market-weighted index, the UPs and DOWNs of the larger company would have twice the impact on the overall stock index–in either UP or DOWN movement–as the smaller company.
Apple had a total market capitalization, at the market close this past Friday, of just over $619 Billion. Some of the other Super-Large US Corporations had Market Capitalization, on Friday, of: Exxon-Mobil $419.5 Billion; Microsoft $261.5 Billion and IBM $235.4 Billion. So, Apple would have 1.67 times the impact of Exxon Mobil and roughly 2.7 times the impact of Microsoft and IBM.
Apple IS a component of the S & P 500, generally referred to as the broad-based index; however, it has not been included in the DJIA. Apple’s size pales in the context of the overall market value of the 500 companies in the S & P Index. In the Dow Jones Industrial Average, however, which is composed of just 30 companies, it would stand out, and further skew the overall impact of the very largest companies–at the very top.
The other question, besides whether to include Apple in the DJIA, is what company to delete–if AAPL is added. Remember that there can only be 30 companies in the Index. Also, the Index is configured so as to have a diverse representation among the various Industrial Sectors. The only other Technology Companies in the DOW are IBM and Microsoft–both long-time components, especially IBM.