The Dow is a unique stock index
The Dow Jones is a bit wacky when it comes to major stock indexes. Firstly, it only contains 30 titles, which means that it is much more exposed to individual farms than the S&P 500 or the Nasdaq Composite. The Dow also has a new approach to weighting. Market cap weighting is the most popular compounding method. Some indices even pursue equal weighting so that smaller stocks can exert the same influence on index returns as larger stocks.
The Dow zigs where the others zag, and its index weighting is based on the stock prices for each constituent stock. This means that stock splits can drastically change the mix without changing the value of the underlying companies. It also means that smaller companies can have a greater influence on index returns, simply because their price per share is high.
As it stands, Boeing currently represents 4.7% of the Dow Jones. Industry is currently the second largest sector in the index, with 17.8% of the total allocation. The technology is 3 percentage points greater. Buying an index fund that tracks the Dow Jones, such as the SPDR Dow Jones Industrial Average ETF Trust (NYSEMKT: DIA), offers diverse exposure, but not to nearly the same degree as other popular indexes.