An index is a statistical measure of the changes in a portfolio of stocks representing a portion of the overall market. It would be too difficult to track every single security trading in the country. To get around this, we take a smaller sample of the market that is representative of the whole. Thus, just as pollsters use political surveys to gauge the sentiment of the population, investors use indexes to track the performance of the stock market. Ideally, a change in the price of an index represents an exactly proportional change in the stocks included in the index.