Are the stocks of competitor companies negatively correlated?
Not especially. It depends on why sales have changed. If it's just consumer demand, that affects everyone in parallel rather than pushing in opposite direactions. If it's changes other than sales, that may have no effect on other companies. If it's because someone introduced the next must-have-it device and they're selling rapidly and drawing customers from the competing brands, maybe. And that's all neglecting the fact that this may already have been incorporated into the competitor's share price long ago, in anticipation of this news. Sorry, but the market just ain't simple. "It is important to first understand that true causation of share price may not relate to historical correlation. Just like with scientific experiments, correlation does not imply causation. But we use stock price correlation to attempt to infer causation, where it is reasonable to do so. And to do that you need to understand that prices change for many reasons; some company specific, some industry specific, some market specific. Companies in the same industry may correlate when that industry goes up or down; companies with the same market may correlate when that market goes up or down. In general, in most industries, it is reasonable to assume that competitor companies have stocks which strongly correlate (positively) with each-other to the extent that they do the same thing.  For a simple example, consider three resource companies: ""Oil Ltd."" [100% of its assets relate to Oil]; ""Oil and Iron Inc."" [50% of its value relates to Oil, 50% to Iron]; and ""Iron and Copper Ltd."" [50% of its value relates to Iron, 50% to Copper]. For each of these companies, there are many things which affect value, but one could naively simplify things by saying ""value of a resource company is defined by the expected future volume of goods mined/drilled * the expected resource price, less all fixed and variable costs"". So, one major thing that impacts resource companies is simply the current & projected price of those resources. This means that if the price of Oil goes up or down, it will partially affect the value of the two Oil companies above - but how much it affects each company will depend on the volume of Oil it drills, and the timeline that it expects to get that Oil. For example, maybe Oil and Iron Ltd. has no currently producing Oil rigs, but it has just made massive investments which expect to drill Oil in 2 years - and the market expects Oil prices to return to a high value in 2 years.  In that case, a drop in Oil would impact Oil Inc. severely, but perhaps it wouldn't impact Oil and Iron Ltd. as much. In this case, for the particular share price movement related to the price of Oil, the two companies would not be correlated. Iron and Copper Ltd. would be unaffected by the price of Oil [this is a simplification; Oil prices impact many areas of the economy], and therefore there would be no correlation at all between this company's shares. It is also likely that competitors face similar markets. If consumer spending goes down, then perhaps the stock of most consumer product companies would go down as well. There would be outliers, because specific companies may still succeed in a falling market, but in generally, there would be a lot of correlation between two companies with the same market.  In the case that you list, Sony vs Samsung, there would be some factors that correlate positively, and some that correlate negatively. A clean example would be Blackberry stock vs Apple stock - because Apple's success had specifically negative ramifications for Blackberry. And yet, other tech company competitors also succeeded in the same time period, meaning they did not correlate negatively with Apple." "In theory, say we had two soft drink companies, and no other existed. On Jan 1, they report they each had 50% market share for the past year. Over the next year, one company's gain is the other's loss. But over the year, for whatever reason, the market has grown 10% (all the stories of bad water helped this), and while the market share ends at 49/51, the 49 guy has improved his margins, and that stock rises by more than the other.  In general, companies in the same industry will be positively correlated, and strongly so.  I offer my ""spreadsheets are your friend"" advice. I took data over the last 10 years for Coke and Pepsi. Easy to pull from various sites, I tend to use Yahoo. In Excel the function CORREL with let you compare two columns of numbers for correlation. I got a .85 result, pretty high. To show how a different industry would have a lower correlation, I picked Intel. Strangely, enough, Intel and Pepsi had a .94 correlation. A coincidence, I suppose, but my point is that you can easily get data and perform your own analysis to better understand what's going on."
"Another factor to consider, beyond the fact that growth and volatility go together, is that the times when many people will need to liquidate their investments will correlate with the times that many other people need to liquidate their investments, and such correlation will push down the immediate value of those investments. While certificates of deposit have penalties for early withdrawal, one can establish up front what the worst-case penalty would be for cashing it in at the most inopportune time.  By contrast, stocks offer no such assurance.  Stocks sometimes have weird downward spikes that may be short-lived, but if life circumstances force one to liquidate stocks during such a downward spike the ""penalty"" can be much larger than on a CD." "GDP being a measurement for an economy's growth and with the stock market being driven (mostly) by company profits you would expect a tight correlation between GDP growth and stock market performance. After all, a growing economy should lead to a corresponding increase in profit right? But the stock market is heavily influenced by investor mentality; irrational exuberant buying and panic selling make the stock market far more volatile than GDP ever can be. Just look at the 2001 bubble and 2008 panic sell-off for famous examples. I feel emerging markets are particularly prone to overly optimistic buying to ""get in"" on the GDP growth followed by overly pessimistic selling when politics get unfavorable. Also keep in mind that GDP measurements are all done after the fact, the growth that is reported has already happened. The stock market might have already expected the reported growth and priced it in. A final point: governments and companies in emerging markets have a reputation (sometimes deserved) of poor governance, think corruption, nepotism etc. So even if the economy grows substantially investors might not believe they can profit from the growth.  P.S. What do you base the ""no great increase"" on? Emerging markets have had a rough decade but that index would have still returned 9% annually if you held it since 2001." "That's a pretty good question for a six-year-old! In addition to the good answers which point out that expectations are priced in, let's deny the premises of the question: Sales do not increase the value of a company; a company could be, for example, losing money on every sale. Share prices are (at least in theory) correlated with profits.  So let's suppose that company X is unprofitable 320 days a year and is relying upon sales in late November and December to be in the black for the year. (Hence ""black Friday"".)  Carefully examine the supposition of this scenario: we have a company that is so unprofitable that it must gamble everything on successfully convincing bargain hunting consumers in a weak economy to buy stuff they don't actually need from them and not a competitor. Why would this inspire investor confidence? There are plenty of companies that fail to meet their sales targets at Christmas, for plenty of reasons." If you want to invest in the Windows Phone, then you go and find out who makes the Windows Phone i.e. Microsoft. Then you go and decide if Windows Phone is successful will the share price of Microsoft go up (own research/deduction) and if you think that the price of Microsoft has a positive correlation with the Windows Phone, then you could buy shares of Microsoft. There is no way to invest directly in individual products on stock exchanges, you are generally investing in the companies that produce them. You find the ticker of a company by googling. NASDAQ: MSFT "I don't mean to be discouraging, but that isn't necessarily always the case. Despite being the larger company, it is possible the other team will have a lower cost, or be more efficient, or depending on how the organizational structure works out may just simply be in a familiar reporting structure that ""trusts"" them more. I think that's why /u/RustbeltRoots suggested it is nearly impossible to tell." "Investors are ""forever"" comparing the prices of stocks to other stocks. As others have pointed out, this is done faster and more frequently nowadays with high-speed computer programs. There may be no ""fresh"" news on stock A, but if there is fresh news on stock B (as there usually is), the news on B affects the COMPARISON with stock A. That could be what causes trading in stock A that has ""no news.""" More questions!  1.) I thought the criticism of the Dow was that it's much smaller than other indexes and thus less representative of the market as a whole?  2,) When you say private investors are you talking about a few specific people? Or anyone who invests at all?  Thanks The top ten holdings for these funds don't overlap by even one stock. It seems to me they are targeting an index for comparison, but making no attempt to replicate a list of holdings as would, say, a true S&P index.