The Dow 30, otherwise referred to as the Dow Jones Industrial Average (DJIA), is one of the major U.S. stock indexes which tracks the stock performance of 30 large, publicly-traded companies in the United States. This index serves as a gauge of the U.S. economy's overall health as it assesses the average share price of all the companies in the index.

The Dow 30 is always changing as the composition of the index is often modified to reflect the changing economic landscape. As the 30 companies in the index are changed and modified, the index is continuously comparing performances of companies in different industries. Consequently, the Dow 30 becomes an even more accurate measure of the economy’s performance.

The method of calculation used to determine the average stock prices of the 30 companies in the index sets the Dow 30 apart from the S&P 500. This index uses a different calculation to industry the performance of its 500 sample companies. While the Dow 30 does not weight the census of its 30 companies in its calculation, the S&P 500 does. In other words, the S&P 500 takes into consideration the market capitalization of the company when it calculates the performance of the sample companies.

Despite its usefulness, the Dow 30 has come under criticism for its lack of diversity. Critics of the Dow believe it fails to represent the U.S. market as the index only takes stock of 30 large-cap companies. As a consequence, the Dow does not capture the fluctuations within mid-cap and small-cap companies. In addition, the Dow 30 economic performance indicator is not as inclusive of new rising companies, which can contribute to the overall growth and success of the U.S. market.

Individuals who wish to invest in the Dow 30 can do so through the SPDR Dow Jones Industrial Average ETF (DIA). The ETF offers investors a practical way of participating in the economic performance of the Dow 30 companies.

In conclusion, the Dow 30, also known as the Dow Jones Industrial Average (DJIA), is a major U.S. stock index which tracks the stock performance of 30 large, publicly-traded companies in the United States. The Dow is often considered to be an accurate measure of the economy’s overall health as it takes the average of one share price of all the companies in the index. While individuals can invest in the Dow through the SPDR Dow Jones Industrial Average ETF (DIA), the Dow 30 has come under criticism for its lack of diversity.