How much of each stock do index funds hold?
In general, the goal of an S&P 500 index fund is to replicate the performance of the S&P 500 Index.  To do this, the fund will buy the same stocks in the same proportions as the weighting of the Index. The S&P 500 Index is free-float capitalization weighted.  This means that the higher capitalization stocks (based on publicly traded shares only) are more heavily weighted and factor into the Index value more heavily than the smaller capitalization stocks, or the stocks that have a smaller publicly traded value.  For example, companies like Apple, ExxonMobil, and Microsoft have a much larger weight in the index value than smaller companies. Alternatively, there are some S&P index funds that are equal-weighted.  In these funds, the managers have chosen to purchase all 500 of the stocks in the index, but in equal proportions instead of the weighted proportions of the index.  These equal-weighted funds will not as closely match the index price as the traditionally weighted index funds.  Instead, they might do better or worse than the index, depending on how the individual stocks do. You'll need to look at the prospectus of the index funds you are interested in to see which approach the fund is taking. An index fund is just copying the definition of an index. The group that defines the index determines how to weight the different parts of the index. The index fund just makes sure they invest the same way the index creator wants.  Think of a non-investment scenario. A teacher can grade tests, quizzes, homework, in-class assignments, research papers. They decide how much weight to give each category and how much weight to give each part of each category. when a student wants to see how they are doing they take the information in the syllabus, and generate a few formulas in a spreadsheet to calculate their current grade. They can also calculate what they need to get on the final exam to get the grade they want. Yes, it depends on the fund it's trying to mirror.  The ETF for the S&P that's best known (in my opinion) is SPY and you see the breakdown of its holdings.  Clearly, it's not an equal weighted index. "Note that an index fund may not be able to precisely mirror the index it's tracking. If enough many people invest enough money into funds based on that index, there may not always be sufficient shares available of every stock included in the index for the fund to both accept additional investment and track the index precisely.  This is one of the places where the details of one index fund may differ from another even when they're following the same index. IDEALLY they ought to deliver the same returns, but in practical terms they're going to diverge a bit. (Personally, as long as I'm getting ""market rate of return"" or better on average across all my funds, at a risk I'm comfortable with, I honestly don't care enough to try to optimize it further. Pick a distribution based on some stochastic modelling tools, rebalance periodically to maintain that distribution, and otherwise ignore it. That's very much to the taste of someone like me who wants the savings to work for him rather than vice versa.)"
How do (index and active) mutual funds trade? Do they buy stocks as soon as a I buy a share in the mutual fund, or do they have fixed times they trade, such as once every week/month/quarter?   Is it theoretical possible for someone to front run mutual funds, if someone holds individual stocks? Let's say an institutional investor creates an order of $100m in a mutual fund, how likely can a broker, which holds a fraction of the fund's portfolio, front run and take advantage of that trade? It is more likely to front run that fund if it's an active small cap fund, but how likely is it to front run trades for index funds? The reports of my death have been greatly exaggerated. - Twain I use index funds in my retirement planning, but don't stick to just S&P 500 index funds. Suppose I balance my money 50/50 between Small Cap and Large Cap and say I have $10,000.  I'd buy $5,000 of an S&P Index fund and $5,000 of a Russell 2000 index fund. Now, fast forward a year.  Suppose the S&P Index fund has $4900 and the Russell Index fund has $5200.   Sell $150 of Russell Index Fund and buy $150 of S&P 500 Index funds to balance.  Repeat that activity every 12-18 months.  This lets you be hands off (index fund-style) on your investment choices but still take advantage of great markets.   This way, I can still rebalance to sell high and buy low, but I'm not stressing about an individual stock or mutual fund choice. You can repeat this model with more categories, I chose two for the simplicity of explaining. During the course of the year, the S&P individual stocks will have some dividends. Not every last stock but a good number of them. Enough that the average dividend for the S&P has been about 2% recently.  So if the S&P index goes up, say 10%, an S&P fund should go up closer to 12%. For a fund holder, you'd normally see a declared dividend and cap gain distribution toward the end of each year. When you hold shares in a 401(k), dividends are reinvested into the fund, usually with no involvement from the members. This isn't totally wrong- there are hedge funds that are long 150% of AUM and short 50%. However, Rentech has said that holding a position for 8 seconds is long for them, so that's not what they're doing. I'd assume the 4X leverage most just refers to option positions that have delta 4 on average. They also may be borrowing money, which they can probably do extremely cheaply since they have a 35 year track record showing they're essentially risk-free. "From How are indexes weighted?: Market-capitalization weighted indexes (or market cap- or cap-weighted   indexes) weight their securities by market value as measured by   capitalization: that is, current security price * outstanding shares.   The vast majority of equity indexes today are cap-weighted, including   the S&P 500 and the FTSE 100. In a cap-weighted index, changes in the   market value of larger securities move the index’s overall trajectory   more than those of smaller ones. If the fund you are referencing is an ETF then there may be some work to do to figure out what underlying securities to use when handling Creation and Redemption units as an ETF will generally have shares created in 50,000 shares at a time through Authorized Participants. If the fund you are referencing is an open-end fund then there is still cash flows to manage in the fund as the fund has create and redeem shares in on a daily basis. Note in both cases that there can be updates to an index such as quarterly rebalancing of outstanding share counts, changes in members because of mergers, acquisitions or spin-offs and possibly a few other factors. How to Beat the Benchmark has a piece that may also be useful here for those indices with many members from 1998: As you can see, its TE is also persistently positive, but if anything   seems to be declining over time. In fact, the average net TE for the   whole period is +0.155% per month, or an astounding +1.88% pa net   after expenses. The fund expense ratio is 0.61% annually, for a   whopping before expense TE of +2.5% annually. This is once again   highly statistically significant, with p values of 0.015 after   expenses and 0.0022 before expenses. (The SD of the TE is higher for   DFSCX than for NAESX, lowering its degree of statistical   significance.) It is remarkable enough for any fund to beat its   benchmark by 2.5% annually over 17 years, but it is downright eerie to   see this done by an index fund. To complete the picture, since 1992 the Vanguard Extended Index Fund   has beaten its benchmark (the Wilshire 4500) by 0.56% per year after   expenses (0.81% net of expenses), and even the Vanguard Index Trust   500 has beaten its benchmark by a razor thin 0.08% annually before   (but not after) expenses in the same period. So what is going on here? A hint is found in DFA's 1996 Reference   Guide: The 9-10 Portfolio captures the return behavior of U.S. small company stocks as identified by Rolf Banz and other academic   researchers. Dimensional employs a ""patient buyer"" discount block   trading strategy which has resulted in negative total trading costs,   despite the poor liquidity of small company stocks. Beginning in 1982,   Ibbotson Associates of Chicago has used the 9-10 Portfolio results to   calculate the performance of small company stocks for their Stocks,   Bonds, Bills, and Inflation yearbook. A small cap index fund cannot possibly own all of the thousands of   stocks in its benchmark; instead it owns a ""representative sample.""   Further, these stocks are usually thinly traded, with wide bid/ask   spreads. In essence what the folks at DFA learned was that they could   tell the market makers in these stocks, ""Look old chaps, we don't have   to own your stock, and unless you let us inside your spread, we'll   pitch our tents elsewhere. Further, we're prepared to wait until a   motivated seller wishes to unload a large block."" In a sense, this   gives the fund the luxury of picking and choosing stocks at prices   more favorable than generally available. Hence, higher long term   returns. It appears that Vanguard did not tumble onto this until a   decade later, but tumble they did. To complete the picture, this strategy works best in the thinnest   markets, so the excess returns are greatest in the smallest stocks,   which is why the positive TE is greatest for the DFA 9-10 Fund, less   in the Vanguard Small Cap Fund, less still in the Vanguard Index   Extended Fund, and minuscule with the S&P500. There are some who say the biggest joke in the world of finance is the   idea of value added active management. If so, then the punch line   seems to be this: If you really want to beat the indexes, then you   gotta buy an index fund." Index funds: Some of the funds listed by US SIF are index funds. ETFs: ETFdb has a list, though it's pretty short at the moment. Depends on the fund. If it's a Target Date fund, which is inherently diversified, this comes down to how much you trust the investment house to not go belly-up. If it's another kind of fund, you need to manage your own duversification and occasional rebalancing.  Most of my money is in index funds (details elsewhere), but that's five or six very different indexes to cover the investment space with the mix of investment types I've selected. And most of it is in a single family of funds, which might be argued to be higher risk than desirable but which has been convenient. Nobody tracks a single company's net assets on a daily basis, and stock prices are almost never derived directly from their assets (otherwise there would be no concept of 'growth stocks').  Stocks trade on the presumed current value of future positive cash flow, not on the value of their assets alone. Funds are totally different.  They own nothing but stocks and are valued on the basis on the value of those stocks.  (Commodity funds and closed funds muddy the picture somewhat, but basically a fund's only business is owning very liquid assets, not using their assets to produce wealth the way companies do.) A fund has no meaning other than the direct value of its assets.   Even companies which own and exploit large assets, like resource companies, are far more complicated than funds: e.g. gold mining or oil extracting companies derive most of their value from their physical holdings, but those holdings value depends on the moving price and assumed future price of the commodity and also on the operations (efficiency of extraction etc.) Still different from a fund which only owns very liquid assets. If you bought 5 shares @ $20 each that would cost you $100 plus brokerage. Even if your brokerage was only $10 in and out, your shares would have to go up 20% just for you to break even. You don't make a profit until you sell, so just for you to break even your shares need to go up to $24 per share. Because your share holding would be so small the brokerage, even the cheapest around, would end up being a large percentage cost of any overall profits. If instead you had bought 500 shares at $20, being $1000, the $20 brokerage (in and out) only represents 2% instead of 20%. This is called economies of scale. The portfolio manager at Value Research Online does this very nicely. It tracks the underlying holdings of each fund, yielding correct calculations for funds that invest across the board. Take a look at the screenshot from my account:  If you have direct equity holdings (e.g., not through a mutual fund), that too gets integrated. Per stock details are also visible. http://mobile.nytimes.com/blogs/economix/2014/04/02/the-many-classes-of-google-stock/ Are you counting both class A and other share classes? I tried direct indexing the S&amp;P500 myself and it was a lot of work. Lots of buys and sells to rebalance, tons of time in spreadsheets running calculations/monitoring etc, dealing with stocks being added or removed from the index, adding money (inflows). Etc. All of the work is the main reason I stopped. I came to realize the 0.05% I pay Vanguard is a great deal. As has been pointed out, one isn't cheaper than the other. One may have a lower price per share than the other, but that's not the same thing. Let's pretend that the total market valuation of all the stocks within the index was $10,000,000. (Look, I said let's pretend.) You want to invest $1,000. For the time being, let's also pretend that your purchasing 0.01% of all the stock won't affect prices anywhere. One company splits the index into 10,000 parts worth $1,000 each. The other splits the same index into 10,000,000 parts worth $1 each. Both track the underlying index perfectly. If you invest $1,000 with the first company, you get one part; if you invest $1,000 with the second, you get 1,000 parts. Ignoring spreads, transaction fees and the like, immediately after the purchase, both are worth exactly $1,000 to you. Now, suppose the index goes up 2%. The first company's shares of the index (of which you would have exactly one) are now worth $1,020 each, and the second company's shares of the index (of which you would have exactly 1,000) are worth $1.02 each. In each case, you now have index shares valued at $1,020 for a 2% increase ($1,020 / $1,000 = 1.02 = 102% of your original investment). As you can see, there is no reason to look at the price per share unless you have to buy in terms of whole shares, which is common in the stock market but not necessarily common at all in mutual funds. Because in this case, both funds track the same underlying index, there is no real reason to purchase one rather than the other because you believe they will perform differently. In an ideal world, the two will perform exactly equally. The way to compare the price of mutual funds is to look at the expense ratio. The lower the expense ratio is, the cheaper the fund is, and the less of your money is being eroded every day in fees. Unless you have some very good reason to do differently, that is how you should compare the price of any investment vehicles that track the same underlying commodity (in this case, the S&P 500). Generally, the answer to the availability of holdings of a given mutual fund on a daily basis is no. Thus, an API is non-existent. The reasons for the lack of transparency on a daily basis is that it could/would impact the portfolio managers ability to trade. While this information would not necessarily permit individuals from front running the fund manager's trades, it does give insight in to the market outlook and strategy the fund is employing. The closest you'll be able to get to obtaining a list of holdings is by reading the most recent annual report and the quarterly filings each fund is required to file with the SEC. Given that a mutual fund manager knows, at the end of the day, precisely how many shares/units/whatever of each investment (stock, equity, etc.) they own, plus their bank balance,  It is calculating this given. There are multiple orders that a fund manager requests for execution, some get settled [i.e. get converted into trade], the shares itself don't get into account immediately, but next day or 2 days later depending on the exchange. Similarly he would have sold quite a few shares and that would still show shares in his account. The bank balance itself will not show the funds to pay as the fund manager has purchased something ... or the funds received as the fund manager has sold something. So in general they roughly know the value ... but they don't exactly know the value and would have to factor the above variables. That's not a simple task when you are talking about multiple trades across multiple shares.