What is the “point” (purpose) of the S&P 500?
I hate to point to Wikipedia as an answer, but it does describe exactly what you are looking for... The S&P 500 is a free-float   capitalization-weighted index   published since 1957 of the prices of   500 large-cap common stocks actively   traded in the United States. The   stocks included in the S&P 500 are   those of large publicly held companies   that trade on either of the two   largest American stock market   exchanges; the New York Stock Exchange   and the NASDAQ. The components of the S&P 500 are   selected by committee... The committee   selects the companies in the S&P 500   so they are representative of the   industries in the United States   economy. In addition, companies that   do not trade publicly (such as those   that are privately or mutually held)   and stocks that do not have sufficient   liquidity are not in the index. The S&P is a capitalization weighted index.  If a stock price goes up, then it comprises more of the total index.  If a stock goes down, it comprises less, and if it goes down too much, the committee will likely replace it. So to answer your question, if one stock were to suddenly skyrocket, nothing would happen beyond the fact that the index was now worth more and that particular stock would now make up a larger percentage of the S&P 500 index.
"They do but you're missing some calculations needed to gain an understanding. Intro To Stock Index Weighting Methods notes in part: Market cap is the most common weighting method used by an index.   Market cap or market capitalization is the standard way to measure the   size of the company. You might have heard of large, mid, or small cap   stocks? Large cap stocks carry a higher weighting in this index. And   most of the major indices, like the S&P 500, use the market cap   weighting method. Stocks are weighted by the proportion of their market cap to the total   market cap of all the stocks in the index. As a stock’s price and   market cap rises, it gains a bigger weighting in the index. In turn   the opposite, lower stock price and market cap, pushes its weighting   down in the index.  Pros Proponents argue that large companies have a bigger effect on the   economy and are more widely owned. So they should have a bigger   representation when measuring the performance of the market. Which is   true. Cons It doesn’t make sense as an investment strategy. According to a market   cap weighted index, investors would buy more of a stock as its price   rises and sell the stock as the price falls. This is the exact   opposite of the buy low, sell high mentality investors should use. Eventually, you would have more money in overpriced stocks and less in   underpriced stocks. Yet most index funds follow this weighting method. Thus, there was likely a point in time where the S & P 500's initial sum was equated to a specific value though this is the part you may be missing here. Also, how do you handle when constituents change over time? For example, suppose in the S & P 500 that a $100,000,000 company is taken out and replaced with a $10,000,000,000 company that shouldn't suddenly make the index jump by a bunch of points because the underlying security was swapped or would you be cool with there being jumps when companies change or shares outstanding are rebalanced? Consider carefully how you answer that question. In terms of histories, Dow Jones Industrial Average and S & P 500 Index would be covered on Wikipedia where from the latter link: The ""Composite Index"",[13] as the S&P 500 was first called when it   introduced its first stock index in 1923, began tracking a small   number of stocks. Three years later in 1926, the Composite Index   expanded to 90 stocks and then in 1957 it expanded to its current   500.[13] Standard & Poor's, a company that doles out financial information and analysis, was founded in 1860 by Henry Varnum Poor. In   1941 Poor's Publishing (Henry Varnum Poor's original company) merged   with Standard Statistics (founded in 1906 as the Standard Statistics   Bureau) and therein assumed the name Standard and Poor's Corporation.   The S&P 500 index in its present form began on March 4, 1957.   Technology has allowed the index to be calculated and disseminated in   real time. The S&P 500 is widely used as a measure of the general   level of stock prices, as it includes both growth stocks and value   stocks. In September 1962, Ultronic Systems Corp. entered into an agreement   with Standard and Poor's. Under the terms of this agreement, Ultronics   computed the S&P 500 Stock Composite Index, the 425 Stock Industrial   Index, the 50 Stock Utility Index, and the 25 Stock Rail Index.   Throughout the market day these statistics were furnished to Standard   & Poor's. In addition, Ultronics also computed and reported the 94 S&P   sub-indexes.[14] There are also articles like Business Insider that have this graphic that may be interesting: S & P changes over the years The makeup of the S&P 500 is constantly changing notes in part: ""In most years 25 to 30 stocks in the S&P 500 are replaced,"" said   David Blitzer, S&P's Chairman of the Index Committee. And while there   are strict guidelines for what companies are added, the final decision   and timing of that decision depends on what's going through the heads   of a handful of people employed by Dow Jones." There is most likely an error in the WSJ's data.  Yahoo! Finance reports the P/E on the Russell 2000 to be 15 as of 8/31/11 and S&P 500 P/E to be 13 (about the same as WSJ). Good catch, though!  E-mail WSJ, perhaps they will be grateful. In addition to the excellent answers here I might suggest a reason for investing in leveraged funds and the original purpose for their existence.  Lets say you run a mutual fund that is supposed to track the performance of the S&P 500.   If you have cash inflows and outflows from your fund due to people investing and selling shares of your fund you may have periods where not all funds are invested appropriately because some of the funds are in cash.  Lets say 98% of your funds are invested in the securities that reflect the stocks in the S&P 500.  You will will miss matching the S&P 500 because you have 2% not invested in some money market account.   If you take 1/3 of the cash balance and invest in a triple leveraged fund or take 1/2 of the funds and invest in a double leveraged fund you will more accurately track the index to which your fund is supposed to track.  I am not sure what percentage mutual fund owners keep in cash but this is one use that I know these ETFs are used for. The difference over time that compounding effects have on leveraged funds is called Beta Slippage. There are many fine articles explaining it at you can find one located at this link. This doesn't really make sense, as the small cap indices have a similar rate of growth as the s&amp;p 500. The global scale has some relevance, but exports are included in GDP as well, so it's not the full story.  A better answer, as is noted in comments lower down, is that the S&amp;P does not **grow** at 6%. It *returns* 6% per year. The dividend yield of the s&amp;p is a bit over 2%. Dividends are not growth, they are simply income; they may also grow themselves, but rarely by 6% per year (in real terms). Additional income that's not paid out in dividends probably amounts to another 1-2%. Growth is the final 1-3%, which makes it roughly match GDP. "It's a matter of social policy. The government wants people to make long term investments because that would lead to other long-term government goals: employment, manufacturing, economical growth in general. While speculative investments and day-trading are not in any way discouraged, investments that contribute to the economy as a whole and not just the investor are encouraged by the lower tax rates on the profits. While some people consider it to be a ""fig leaf"", I consider these people to be populists and dishonest. Claiming that long term social goals are somehow bad is hypocrisy. Claiming that short-term trading contributes to the economy as a whole is a plain lie." Value investing is just an investment strategy, it's an alternative to technical investing. Buffet made money picking stocks.  It's not obvious how that adds value, but it does. Everything about the stock market is ultimately about IPOs. Without active trading, of stocks after issue, no one would buy at the IPO.  The purpose of an IPO is to finance the long-term growth of a business, which is the point in the process where the value to the people gets created.  There is a group of elites that needs to be dealt with, you're correct, but I worry that your definition of this group is overly broad. "What does ""points"" mean In any stock market, there are certain stocks that go up and certain stocks that go down. Hence if we want to find the generic health of stock market, i.e. on an average is it going up or down, we have no means to find out.  A practise that has evolved over the years is take a set of companies and find if on average they have gone up or gone down. In very simple terms say in 1970 I take the Market Capitalization of a set of 50 companies, lets say its value is ""X"". I would now call this index as value of 100. Now after a month if the Market Capitalization is 2X, the index value would be 200. After another month if the Market Capitalization come down to 1.5X, then index value would be 150. So essentially now one is able to get the general trend more easily. S&P is an index of Select 500 companies based on various parameters. So in isolation 2000 does not mean anything. However as a comparison it does give quite a bit of insight. Note there are various adjustments made to factor, i.e. certain companies go bankrupt or are not doing well are removed from Index, share splits, mergers, etc. This ensure that the Index is neutral and does not show unwarranted spikes." "Following comments to your question here, you posted a separate question about why SPY, SPX, and the options contract don't move perfectly together.  That's here Why don't SPY, SPX, and the e-mini s&p 500 track perfectly with each other? I provided an answer to that question and will build on it to answer what I think you're asking on this question.  Specifically, I explained what it means that these are ""all based on the S&P.""  Each is a different entity, and different market forces keep them aligned.  I think talking about ""technicals"" on options contracts is going to be too confusing since they are really a very different beast based on forward pricing models, so, for this question, I'll focus on only SPY and SPX. As in my other answer, it's only through specific market forces (the creation / redemption mechanism that I described in my other answer), that they track at all.  There's nothing automatic about this and it has nothing to do with some issuer of SPY actually holding stock in the companies that comprise the SPX index. (That's not to say that the company does or doesn't hold, just that this doesn't drive the prices.)  What ever technical signals you're tracking, will reflect all of the market forces at play.  For SPX (the index), that means some aggregate behavior of the component companies, computed in a ""mathematically pure"" way.  For SPY (the ETF), that means (a) the behavior of SPX and (b) the behavior of the ETF as it trades on the market, and (c) the action of the authorized participants.  These are simply different things. Which one is ""right""?  That depends on what you want to do.  In theory you might be able to do some analysis of technical signals on SPY and SPX and, for example, use that to make money on the way that they fail to track each other.  If you figure out how to do that, though, don't post it here.  Send it to me directly. :)"