Relationship between liquidity and an efficient market
"Liquidity is highly correlated to efficiency primarily because if an asset's price is not sampled during the time of a trade, it's price is unknown therefore inefficient.  Past prices can be referenced, but they are not the price of the present.  Prices of substitutes are even worse. SPY is extremely efficient for an equity.  If permitted, it could easily trade with much lower ticks and still have potential for a locked market. Ideal exchange An ideal exchange has no public restrictions on trade.  This is not to say that private restrictions would need to be put in place for various reasons, but one would only do that if it were responsible for its own survival instead of being too big to fail. In this market, trades would be approximately continuous for the largest securities and almost always locked because of continuous exchange fee competition with ever dropping minimum ticks. A market that can provide continuous locked orders with infinite precision is perfectly efficient from the point of view of the investor because the value of one's holdings are always known. EMH In terms of the theory the Efficient Market Hypothesis this is irrelevant to the rational investor. The rational investor will invest in the market at large of a given asset class, only increasing risk as wealth increases thus moving to more volatile asset classes when the volatility can be absorbed by excess wealth. Here, liquidity is also helpful, the ""two heads are better than one"" way of thinking.  The more invested in an asset class, the lower the class's variance and vice versa.  Bonds, the least variant, dwarf equities which dwarf options, all in order of the least variance.  Believe it or not, there was a day when bonds were almost as risky as equities. For those concerned with EMH, liquidity is also believed to increase efficiency in some forms because liquidity is proportional to the number of individuals invested thus reducing the likelihood of an insufficient number of participants. External inefficiency In the case of ETFs that do not perfectly track their underlying index less costs at all times between index changes, this is because they are forbidden from directly trading in the market on their own behalf. If they were allowed and honest, the price would always be perfect and much more liquid than it otherwise should be since the combined frequency of all index members is much higher than any one alone. If one was dishonest, it would try to defraud with higher or lower numbers; however, if insider trading were permitted, both would fail due to the prisoner's dilemma that there is no honor among thieves.  Here, the market would detect the problem much sooner because the insiders would arbitrage the false price away.   Indirect internal efficiency Taking emerging market ETFs as an example, the markets that those are invested into are heavily restricted, so their ETF to underlying price inefficiencies are more pronounced even though the ETFs are actually working to make those underlying markets more efficient because a price for them altogether is known."
"Generally investing in index-tracking funds in the long term poses relatively low risk (compared to ""short term investment"", aka speculation). No-one says differently. However, it is a higher risk than money-market/savings/bonds. The reason for that is that the return is not guaranteed and loss is not limited. Here volatility plays part, as well as general market conditions (although the volatility risk also affects bonds at some level as well). While long term trend may be upwards, short term trend may be significantly different. Take as an example year 2008 for S&P500. If, by any chance, you needed to liquidate your investment in November 2008 after investing in November 1998 - you might have ended up with 0 gain (or even loss). Had you waited just another year (or liquidated a year earlier) - the result would be significantly different. That's the volatility risk. You don't invest indefinitely, even when you invest long term. At some point you'll have to liquidate your investment. Higher volatility means that there's a higher chance of downward spike just at that point of time killing your gains, even if the general trend over the period around that point of time was upward (as it was for S&P500, for example, for the period 1998-2014, with the significant downward spikes in 2003 and 2008). If you invest in major indexes, these kinds of risks are hard to avoid (as they're all tied together). So you need to diversify between different kinds of investments (bonds vs stocks, as the books ""parrot""), and/or different markets (not only US, but also foreign)." "Exchange A has 100 shares of a stock at $10, the next 100 shares cost $10.01. Exchange B has the same pricing structure.  A fund manager wants to buy 200 shares of the stock, and decides that buying 100 shares at $10 from each exchange will be cheaper than staying in one exchange and paying $10.01 for the second half of his order.  The manager places two separate orders. Let's say the first order reaches exchange A, and the trade executes at $10. Traders (algorithms) on exchange B see this happen, and adjust their price up to $10.01 accordingly. Now, when the manager's order reaches exchange B, there will no longer be any shares trading at $10.  Some people say that this is front running, but if the manager only wanted 100 shares, the price would have still shifted. Some say this creates a more efficient market with tighter spreads due to the decreased risk to the market maker, but it also means the aggregate bid-ask offers across multiple exchanges are not necessarily accurate, creating a ""false liquidity"". You can decide for yourself whether or not this is a good thing." What you have to remember is that Options are derivatives of another asset like stocks for example.  The price of the Option is derived from the price of the underlying. If the underlying is a stock for example, as the price of the stock moves up and down during the trading day, so will the Market Maker's fair value for the Option.  As Options are usually less liquid than the underlying stock, Market Makers are usually more active in 'Providing a Market' with Options. Thus if you place a limit order half way between the current Bid and Ask and the underlying stock price moves towards your limit order, the Market Maker will do their job and 'Provide a Market' at that price, thus executing your order. &gt; Why would the executives take accept a salary at 80% of the market wage when they could just get a job at 100% of the market wage.  This may be true for managers, but research has found little to no evidence of a relationship between executive pay and performance.  There is no market for executives.  They're all on each others boards.  They get much of their pay in stock options, diluting the wealth of those shareholders you're fighting for, without telling shareholders how much they're costing them. In my experience thanks to algorithmic trading the variation of the spread and the range of trading straight after a major data release will be as random as possible, since we live in an age that if some pattern existed at these times HFT firms would take out any opportunity within nanoseconds. Remember that some firms write algorithms to predict other algorithms, and it is at times like those that this strategy would be most effective. With regards to my own trading experience I have seen orders fill almost €400 per contract outside of the quoted range, but this is only in the most volatile market conditions. Generally speaking, event investing around numbers like these are only for top wall street firms that can use co-location servers and get a ping time to the exchange of less than 5ms. Also, after a data release the market can surge/plummet in either direction, only to recover almost instantly and take out any stops that were in its path. So generally, I would say that slippage is extremely unpredictable in these cases( because it is an advantage to HFT firms to make it so ) and stop-loss orders will only provide limited protection. There is stop-limit orders( which allow you to specify a price limit that is acceptable ) on some markets and as far as I know InteractiveBrokers provide a guaranteed stop-loss fill( For a price of course ) that could be worth looking at, personally I dont use IB.  I hope this answer provides some helpful information, and generally speaking, super-short term investing is for algorithms. "This is a very important question and you will find arguments from both sides, in part because it is still understudied. Ben Golub, Economics Ph.D., from Stanford answers ""Is high-frequency trading good for the economy?"" on Quoram quite well.  This is an important but understudied question. There are few   published academic studies on it, though several groups are working on   the subject. You may be interested in the following papers: http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1569067 http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1361184 These document some of the phenomena that arise in high frequency   trading, from a theoretical and an empirical perspective. However, a   full equilibrium analysis of the unique features of high frequency   trading is still missing, and until it is done, all our answers will   be kind of  tentative. Nevertheless, there are some obvious things one can say. Currently,   high frequency traders are competing to locate physically closer and   closer to exchanges, because milliseconds matter. Thus, large amounts   of money are being spent to beat other market makers by tiny fractions   of a second. Once many firms make these investments, the market looks   like it did before in terms of competition and prices, but is a tiny   bit faster. This investment is unlikely to be socially efficient: that   is, the users of  the market don't actually benefit from the fact that   their trades are executed half a millisecond faster -- certainly not   enough to cover all the investment that went into making that happen. Some people who study the issue believe that high frequency trading   (HFT) actually exacerbates market volatility; some plots to this   effect are found in the second paper linked above. There is certainly   no widely accepted theory that says faster trading technology   necessarily increases efficiency, and it is easy to think of   algorithms that can make money (at least in the short run) but hurt   most other investors, as well as the informational value of the   market. One caution is that some of the complaining about HFT comes from those   who lose when HFT gets better -- old-style market makers. They   certainly have an incentive to make HFT out to be very bad. So some   complaints about the predatory nature of HFT should be taken with a   grain of salt. There is no strong economic consensus about the value   of this activity. For what it's worth, my personal impression is that   this is more bad than good. I'll post an update here as more   definitive research comes out. You can also find a debate on High-frequency trading from the Economist which gives both sides of the argument. In conclusion: Regardless of how you feel about HFT it seems like it's here to stay and won't be leaving in the foreseeable future. So the debate will rage on... Additional resource you may finding interesting: Europe Begins Push To Ban HFT High Frequency Trading Discussion On CNBC Should High Frequency Trading (HFT) be banned ?" "No, but it is certainly a possibility. the efficient market hypothesis would say that this means that the market perceives the present value of all future earning as negative. These earnings might take the form of a writedown of assets at some point. (Companies carry a goodwill asset that is generally imaginary.  They book that asset when they buy companies for more than they are worth.) It would be as if PRUN was a stock tracking my life.  If I bought my house in 2006 for $1 million cash. I might have a book value of $1 million.  However, PRUN might trade at $500k because the market knows that my asset isn't really worth $1 million and at some point my earnings will take a hit to reflect that. It might also mean that future ""real"" earnings ""ie actual profit and loss on sales"" are going to be negative. This would mean bankruptcy is more likely." Theoretically, it's a question of rate of return. If a desired or acceptable rate of return for market makers' capital is X, and X is determined by the product of margin & turnover then higher turnover means lower margin for a constant X.  Margin, in the case of trading, is the bid/ask spread, and turnover, in the case of trading, is volume. Empirically, it has been noted in the last markets still offering such wide-varying evidence, equity options: http://faculty.baruch.cuny.edu/lwu/890/mayhew_jf2002.pdf