Technical Analysis: the concepts of overbought / oversold don't make sense
"Some technical indicators (e.g. Williams %R) indicate whether the market is overbought or oversold.   ... Every time a stock or commodity is bought, it is also sold. And vice versa. So how can anything ever be over-bought or over-sold? But I'm sure I'm missing something. What is it? You're thinking of this as a normal purchase, but that's not really how equity markets operate. First, just because there are shares of stock purchased, it doesn't mean that there was real investor buyer and seller demand for that instrument (at that point in time).  Markets have dedicated middlemen called Market Makers, who are responsible to make sure that there is always someone to buy or sell; this ensures that all instruments have sufficient liquidity.  Market Makers may decide to lower their bid on a stock based on a high number of sellers, or raise their ask for a high number of buyers. During an investor rush to buy or sell an instrument (perhaps in response to a news release), it's possible for Market Makers to accumulate a large number of shares, without end-investors being involved on both sides of the transaction.  This is one example of how instruments can be over-bought or over-sold. Since Williams %R creates over-bought and over-sold signals based on historical averages of open / close prices, perhaps it's better to think of these terms as ""over-valued"" and ""under-valued"".  Of course, there could be good reason for instruments to open or close outside their expected ranges, so Williams %R is just a tool to give you clues... not a real evaluation of the instrument's true value." You are right that every transaction involves a seller and a buyer. The difference is the level of willingness from both parties.  Overbought and oversold, as I understand them (particularly in the context of stocks), describe prolonged price increase (overbought, people are more willing to buy than sell, driving price up) and price decrease (oversold, people are more willing to sell than buy, driving price down). "Investopedia's explanation of overbought: An asset that has experienced sharp upward movements over a very short period of time is often deemed to be overbought. Determining the   degree in which an asset is overbought is very subjective and can   differ between investors. Technicians use indicators such as the relative strength index, the stochastic oscillator or the money flow index to identify securities   that are becoming overbought. An overbought security is the opposite of one that is oversold. Something to consider is the ""potential buyers"" and ""potential sellers"" of a stock. In the case of overbought, there are many more buyers that have appeared and driven the price to a point that may be seen as ""unsustainably high"" and thus may well come down soon if one looks at the first explanation. For oversold, consider the flip side of this. A real life scenario here would be to consider airline tickets where a flight may be ""overbooked"" that could also be seen as ""oversold"" in that more tickets were sold than seats that are available and thus people will be bumped as not all tickets can be honored in this case. For a stock scenario of ""oversold"" consider how IPOs work where several buyers have to exist to buy the shares so the investment bank isn't stuck holding them which sends up the price since the amount wanted by the buyers may be more than what can be sold. The price shifts in bringing out more of one side than the other is the point you are missing. In shifting the price up, this attracts more sellers to satisfy the buyers. However, if there is a surge of buyers that flood the market, then there could be a perception that the security is overbought in the sense that there may be few buyers left for the security and thus the price may fall in the near term. If the price is coming down, this attracts more buyers to achieve the other side. The potential part is what you don't see and I wonder if you can imagine this part of the market. The airline example I give as an example as you don't seem to think either side of buying or selling can be overloaded. In the case of an oversold flight, there were more seats sold than available so yes it is possible. Stocks exist in finite quantities as there are only X shares of a company trading at any one time if you look into the concept of a float."
"What most of these answers here seem to be missing is that a stock ""price"" is not exactly what we typically expect a price to be--for example, when we go in to the supermarket and see that the price of a gallon of milk is $2.00, we know that when we go to the cash register that is exactly how much we will pay.  This is not, however, the case for stocks. For stocks, when most people talk about the price or quote, they are really referring to the last price at which that stock traded--which unlike for a gallon of milk at the supermarket, is no guarantee of what the next stock price will be.  Relatively speaking, most stocks are extremely liquid, so they will react to any information which the ""market"" believes has a bearing on the value of their underlying asset almost (if not) immediately.  As an extreme example, if allegations of accounting fraud for a particular company whose stock is trading at $40 come out mid-session, there will not be a gradual decline in the price ($40 -> $39.99 -> $39.97, etc.)-- instead, the price will jump from $40 to say, $20.  In the time between the the $40 trade and the $20 trade, even though we may say the price of the stock was $40, that quote was actually a terrible estimate of the stock's current (post-fraud announcement) price. Considering that the ""price"" of a stock typically does not remain constant even in the span of a few seconds to a few minutes, it should not be hard to believe that this price will not remain constant over the 17.5 hour period from the previous day's close to the current day's open.  Don't forget that as Americans go to bed, the Asian markets are just opening, and by the time US markets have opened, it is already past 2PM in London.  In addition to the information (and therefore new knowledge) gained from these foreign markets' movements, macro factors can also play an important part in a security's price-- perhaps the ECB makes a morning statement that is interpreted as negative news for the markets or a foreign government before the US markets open.  Stock prices on the NYSE, NASDAQ, etc. won't be able to react until 9:30, but the $40 price of the last trade of a broad market ETF at 4PM yesterday probably isn't looking so hot at 6:30 this morning... don't forget either that most individual stocks are correlated with the movement of the broader market, so even news that is not specific to a given security will in all likelihood still have an impact on that security's price. The above are only a few of many examples of things that can impact a stock's valuation between close and open: all sorts of geopolitical events, announcements from large, multi-national companies, macroeconomic stats such as unemployment rates, etc. announced in foreign countries can all play a role in affecting a security's price overnight. As an aside, one of the answers mentioned after hours trading as a reason--in actuality this typically has very little (if any) impact on the next day's prices and is often referred to as ""amateur hour"", due to the fact that trading during this time typically consists of small-time investors.  Prices in AH are very poor predictors of a stock's price at open." "Isn't this effectively saying that the market responds principally to itself, and not to either economic fundamentals, or the profitability of the underlying companies.  If so, the market as a ""price discovery mechanism"" is broken, and investors would be wise to do their own research." That's exactly what immunofort is saying, and it lines up with established financial theory (well, some of it).  The general argument is that stocks are priced according to what risks they're exposed to.  Several (although not all) of the major financial economics theories propose (and find empirical evidence for this) that the market as whole is a risk factor, so individual stocks would be priced in part based on how correlated they are with the market.  Strictly speaking, the risk free rate is purely theoretical and can't be directly observed, but the US T-bill yield is usually considered to be a pretty good proxy for it. "@jidugger mostly got it right. It basically mean that past performance of a stock, or a basket of stocks, are not at all useful when trying to predict its future. There is no proven correlation between past and future performance. If there was such a correlation, that was ""proven"" or known, then investors would quickly exploit this correlation by buying or selling this stock, thus nullifying the prediction. It doesn't mean the specific individuals cannot predict the future stock market - hell, if I set up 2^100 different robots, where every robots gives a different series of answers to the 100 questions ""how will stock X do Y days from now"" (for 1<=Y<=100), then one of those robots would be perfectly correct. The problem is that an outside observer has no way of knowing which of the predictor robots is right. To say that stock is memoryless strikes me as not quite right -- to   the extent that stocks are valued based on earnings, much of what we   infer about future earnings relies on past and present earnings. To put it another way - you have $1000 now, and need to decide whether to invest in a particular stock, or a stock index. The ""memoryless"" property means that no matter how many earning reports you view ... by the time you see them, the stock price already accounts for them, so they're not useful to you. If the earning reports are positive, the stock is already ""too high"" because people bought it before you did. So on average, you can't use this information to predict the stock's future performance, and are better off investing in an index fund (unless you desire extra risk that doesn't come with more profitability)." "I'm not sure the term actually has a clear meaning. We can think of ""what does this mean"" in two ways: its broad semantic/metaphorical meaning, and its mechanical ""what actual variables in the market represent this quantity"". Net buying/selling have a clear meaning in the former sense by analogy to the basic concept of supply and demand in equilibrium markets. It's not as clear what their meaning should be in the latter sense. Roughly, as the top comment notes, you could say that a price decrease is because of net selling at the previous price level, while a price rise is driven by net buying at the previous price level. But in terms of actual market mechanics, the only way prices move is by matching of a buyer and a seller, so every market transaction inherently represents an instantaneous balance across the bid/ask spread. So then we could think about the notion of orders. Actual transactions only occur in balance, but there is a whole book of standing orders at various prices. So maybe we could use some measure of the volume at various price levels in each of the bid/ask books to decide some notion of net buying/selling. But again, actual transactions occur only when matched across the spread. If a significant order volume is added on one side or the other, but at a price far away from the bid/offer - far enough that an actual trade at that price is unlikely to occur - should that be included in the notion of net buying/selling? Presumably there is some price distance from the bid/offer where the orders don't matter for net buying/selling. I'm sure you'd find a lot of buyers for BRK.A at $1, but that's completely irrelevant to the notion of net buying/selling in BRK.A. Maybe the closest thing I can think of in terms of actual market mechanics is the comparative total volumes during the period that would still have been executed if forced to execute at the end of period price. Assuming that traders' valuations are fixed through the period in question, and trading occurs on the basis of fundamentals (which I know isn't a good assumption in practice, but the impact of price history upon future price is too complex for this analysis), we have two cases. If price falls, we can assume all buyers who executed above the last price in the period would have happily bought at the last price (saving money), while all sellers who executed below the last price in the period would also be happy to sell for more. The former will be larger than the latter. If the price rises, the reverse is true." What it is trying to describe is the psychology around the current price of the stock.  In candlestick charts for example, if you get what is called a Bearish Engulfing Candle (where the open is higher than the previous day's close and the close is lower than the previous day's open) at the top of an uptrend, this could mean that the top may have been reached and the bears are taking over the bulls. A Bearish Engulfing candle is seen as a bearish reversal pattern, as the bulls start the day by opening the stock at a higher price than yesterday's close, but by the end of the day the bears have taken over as the price drops below yesterday's open. This reversal pattern can be even more pronounced and effective if it coincides with other chart indicators, such as an overbought momentum indicator. If you want to learn more look up about the Psychology of the market and Candlestick Charting. Who's to say it wasn't priced into the markets, at least to some degree? Without any information on the behaviour of holders pre-expiry, no one can know if they've been shorting the stock in advance of selling on expiry day. And with the float being such a small proportion of the total issuance, there's always the risk of sudden fluctuations picking up big momentum - which could easily explain the 7% drop on expiry day. Add into all this uncertainty, the usual risks of shorting (e.g. limited upside, unlimited downside), and the observed phenomena aren't by any means killer blows of the Efficient Market Hypothesis. That's not to say that such evidence doesn't necessarily exist - just that this isn't it. I find the higher estimates a bit unbelievable. A big part of my job is liability valuation and small assumption changes can have a huge impact on results.  They may be right (future) dollar value wise but the proper way to think about this stuff is in present value terms.  This could actually be a really interesting study - you all just gave me a great idea for a potential masters thesis :) "What you are seeking is termed ""Alpha"", the mispricing in the market. Specifically, Alpha is the price error when compared to the market return and beta of the stock. Modern portfolio theory suggests that a portfolio with good Alpha will maximize profits for a given risk tolerance. The efficient market hypotheses suggests that Alpha is always zero. The EMH also suggests that taxes, human effort and information propagation delays don't exist (i.e. it is wrong). For someone who is right, the best specific answer to your question is presented Ben Graham's book ""The Intelligent Investor"" (starting on page 280). And even still, that book is better summarized by Warren Buffet (see Berkshire Hathaway Letters to Shareholders). In a great disservice to the geniuses above it can be summarized much further: closely follow the company to estimate its true earnings potential... and ignore the prices the market is quoting. ADDENDUM: And when you have earnings potential, calculate value with: NPV = sum(each income piece/(1+cost of capital)^time) Update: See http://finance.fortune.cnn.com/2014/02/24/warren-buffett-berkshire-letter/ ""When Charlie Munger and I buy stocks..."" for these same ideas right from the horse's mouth" "Since doodle77 handled arbitrage, I'll take goodwill.  Goodwill is an accounting term that acts much like a ""plug"" account: you add/subtract to it the amount that makes everything balance.  In the case of goodwill, it generally only applies to mergers &amp; acquisitions.  The theory (and justification) is this: firms buy other firms at prices other than the market price (usually higher), and it is assumed that this is because the acquirer values its acquisition more than other people do.  But whether you use historical prices or market prices when you add (subtract) assets and liabilities to to (from) your balance sheet, this will never add up, because you paid more (less) than the assets are worth in the market, so more (less) cash flowed out than assets flowed in.  The difference goes into the goodwill account, so firms with a large goodwill account are ones that have made lots of acquisitions." Graphs are nothing but a representation of data. Every time a trade is made, a point is plotted on the graph. After points are plotted, they are joined in order to represent the data in a graphical format.  Think about it this way.  1.) Walmart shuts at 12 AM. 2.)Walmart is selling almonds at $10 a pound.  3.) Walmart says that the price is going to reduce to $9 effective tomorrow. 4.) You are inside the store buying almonds at 11:59 PM.  5.) Till you make your way up to the counter, it is already 12:01 AM, so the store is technically shut. 6.) However, they allow you to purchase the almonds since you were already in there.  7.) You purchase the almonds at $9 since the day has changed. 8.) So you have made a trade and it will reflect as a point on the graph. 9.) When those points are joined, the curves on the graph will be created. 10.) The data source is Walmart's system as it reflects the sale to you. ( In your case the NYSE exchange records this trade made). Buying a stock is just like buying almonds. There has to be a buyer. There has to be a seller. There has to be a price to which both agree. As soon as all these conditions are met, and the trade is made, it is reflected on the graph. The only difference between the graphs from 9 AM-4 PM, and 4 PM-9 AM is the time. The trade has happened regardless and NYSE(Or any other stock exchange) has recorded it! The graph is just made from that data. Cheers. It is known as the range or the price spread of the stock. You can read more about it here http://www.investopedia.com/terms/r/range.asp "It means price movements in the past do not affect price movements in the future. Think of the situation of a coin, if you flip it once, and then you flip it a second time, the results are independent of each other. If the first time, you flipped a HEAD, it does not mean that the coin will remember it, and produce a TAIL the second time. This is the meaning of ""memoryless"". FYI, stock markets are clearly not memoryless. It is just an assumption for academic purposes." "I think that ""memoryless"" in this context of a given stock's performance is not a term of art. IMO, it's an anecdotal concept or cliche used to make a point about holding a stock. Sometimes people get stuck... they buy a stock or fund at 50, it goes down to 30, then hold onto it so they can ""get back to even"". By holding the loser stock for emotional reasons, the person potentially misses out on gains elsewhere." Nominal. What you say is true, but I'm guessing it would be too complicated to modelate. Plus, a shareholder of a very large company would not necessarily experience said loss if he/she sells the stock in small chunks at a time.