Average Price of a Stock
Edit3: Regarding the usefulness of the bare number itself, it is not useful unless, for example, an employer uses that average in the computation of how many options the employer grants to the employee as part of the compensation paid. One of my employers used just such an average. What is far more common is to use two or more moving averages, of different periods, plotted on a chart.  My original response continues below... Assuming there are 252 trading days a year, the following chart does what you have done but with a moving average: AAPL on Stockcharts.com  Edit: BTW, I looked up the number of Federal holidays, there are 9. The average year has 365.2422 days. 365.2422 × 5/7 = 260.8873. Subtract 9 and you get 251.8873 trading days in the average year. So 252 is a better number for the SMA than 250 if you want to average a year.  Edit2: Here is the same chart with more than one average included: AAPL chart w/indicators That metric is not very useful for anything other than very extremely long trading periods. Most strategies or concerned with price movement over much shorter time frames, 15 mins, 1 hr, 4 hr, daily, weekly, monthly. The MA or moving average is a trend following lagging indicator used to smooth out price fluctuations and more accurately reflect the price of trading instrument such as a stock (AAPL), commodity, or currency pair. Traders are generally concerned with current market trends and price action of the instrument they are trading. As such, an extremely long MA (average daily price, over a period of 365 days) are generally not that important. "I would have to disagree with the other responders. In technical analysis of stock charts, various short and long term moving averages are used to give an indication of the trend of the stock in the short and long term, as compared to the current price. I would prefer to use the term moving average (MA) rather than average as the MA is recalculated every day (or at appropriate frequencies for your data) on the period you are using. I would also expand on the term ""moving average"". There are two that are commonly used Going back to the question, of the value of this number, For example if the current price is above the 200 day EWMA and also above the 30 day EWMA, then the stock is broadly trending upwards. Conversely if current price is below the 200 day EWMA and also below the 30 day EWMA, then the stock is broadly trending downwards. These numbers are chosen on the basis of the market you are trading in, the volatility and other factors. For another example of how a number of moving averages are used together, please have a read of Daryl Guppy's Multiple Moving Average, though this does not use moving averages as large as 200 days."
If a stock that makes up a big part of the Dow Jones Industrial   Average decided to issue a huge number of additional shares, that will   make the index go up. At least this is what should happen, since an   index is basically a sum of the market cap of the contributing   companies. No, indices can have various weightings. The DJIA is a price-weighted index not market-cap weighted. An alternative weighting besides market-cap and price is equal weighting. From Dow Jones: Dow Jones Industrial Average™. Introduced in May 1896, the index, also   referred to as The Dow®, is a price-weighted measure of 30 U.S.   blue-chip companies. Thus, I can wonder what in the new shares makes the index go up? If a stock is split, the Dow divisor is adjusted as one could easily see how the current Dow value isn't equal to the sum or the share prices of the members of the index. In other cases, there may be a dilution of earnings but that doesn't necessarily affect the stock price directly as there may be options exercised or secondary offerings made. SO if the index, goes up, will the ETF DIA also go up automatically   although no additional buying has happened in the ETF itself? If the index rises and the ETF doesn't proportionally, then there is an arbitrage opportunity for someone to buy the DIA shares that can be redeemed for the underlying stocks that are worth more in this case. Look at the Creation and Redemption Unit process that exists for ETFs. Would you mind adding where that additional value comes from, if not from the losses of other investors?  You asked this in a comment, but it seems to be the key to the confusion.   Corporations generate money (profits, paid as dividends) from sales.  Sales trade products for money.  The creation of the product creates value.  A car is worth more than General Motors pays for its components and inputs, even including labor and overhead as inputs.  That's what profit is:  added value.  The dividend is the return that the stock owner gets for owning the stock.   This can be a bit confusing in the sense that some stocks don't pay dividends.  The theory is that the stock price is still based on the future dividends (or the liquidation price, which you could also consider a type of dividend).  But the current price is mostly based on the likelihood that the stock price will increase rather than any expected dividends during ownership of the stock.   A comment calls out the example of Berkshire Hathaway.  Berkshire Hathaway is a weird case.  It operates more like a mutual fund than a company.  As such, investors prefer that it reinvest its money rather than pay a dividend.  If investors want money from it, they sell shares to other investors.  But that still isn't really a zero sum game, as the stock increases in value over time.   There are other stocks that don't pay dividends.  For example, Digital Equipment Corporation went through its entire existence without ever paying a dividend.  It merged with Compaq, paying investors for owning the stock.   Overall, you can see this in that the stock market goes up on average.  It might have a few losing years, but pick a long enough time frame, and the market will increase during it.  If you sell a stock today, it's because you value the money more than the stock.  If it goes up tomorrow, that's the buyer's good luck.  If it goes down, the buyer's bad luck.  But it shouldn't matter to you.  You wanted money for something.  You received the money.   The increase in the stock market overall is an increase in value.  It is completely unrelated to trading losses.  Over time, trading gains outweigh trading losses for investors as a group.  Individual investors may depart from that, but the overall gain is added value.   If the only way to make gains in the stock market was for someone else to take a loss, then the stock market wouldn't be able to go up.  To view it as a zero sum game, we have to ignore the stocks themselves.  Then each transaction is a payment (loss) for one party and a receipt (gain) for the other.  But the stocks themselves do have value other than what we pay for them.  The net present value of of future payments (dividends, buyouts, etc.) has an intrinsic worth.  It's a risky worth.  Some stocks will turn out to be worthless, but on average the gains outweigh the losses. "A key principle of economics is: Sunk costs are irrelevant. You bought the stock at 147 and it has now fallen to 144. That's too bad. This has nothing to do with whether it is wise or foolish to buy shares at 144. The only relevant thing to consider is: Do I expect the stock to go up or down from 144? You have lost $3 per share on the original buy. Buying more shares will not ""reduce your loss"" in any way. Suppose you bought 100 shares at 147. The price then drops to 144. You have lost $3 per share, or $300 total. You buy another 50 more shares at 144. The price stays at 144. So your average purchase price is now (147 x 100 + 144 x 50) / 150 = 146. So I guess you could say that your ""average loss per share"" is now only $2. But it's $2 x 150 shares instead of $3 x 100 shares. You still lost $300. You didn't reduce your loss by a penny. Maybe it made you feel better that you reduced your average loss per share, but this is just an arithmetic game. If you believe that the stock will continue to drop, than buying more shares just means you will lose even more money. Your average loss per share may go down, but you're just multiplying that average by more and more shares. Of course if you believe that the stock is now at an unjustifiably low price and it will likely go back up, then sure, buy. If you buy at 144 and it goes back up to 147, then you'll be making $3 per share on the new shares you purchased. But I repeat, whether or not you buy more shares should have nothing to do with your previous buy. Buy more shares if you think the price will go up from the present price; don't buy more shares if you don't think it will go up. The decision should be exactly the same as if you had never previously bought shares. (I'm assuming here that you are a typical small investor, that you not buying enough shares to have any significant effect on the market, nor that you are in a position to buy enough shares to take control of the company.)" There are ETF funds that only purchase preferred stock from banks. I have one that pays a monthly dividend of a little under 6% per year. That means that it pays just under 0.5% every month. The purchase price of this stock just slowly goes up and up.  You can do a whole lot better than 2% per year.  The crux of the issue, as I understand it, is the lousy 2% interest she is getting. My point is that you can do a lot better than 2%.  An ETF is not a scam. The price has stability and slow growth because it buys preferred stock from banks.  http://www.marketwatch.com/investing/Fund/PGF?countrycode=US http://stockcharts.com/h-sc/ui?s=PGF&p=D&yr=2&mn=3&dy=0&id=p52078664654 Yes, she should invest. My answer is yes because 2% ROI  is a lousy return and she can do better.  Looking at the 200 day moving average, the price goes from 15.25 in May of 2014 to 17.95 in Dec of 2015. That, in price appreciation alone, is a 17.7% increase. Add on top of that a 0.5% increase per month and you get a stellar 27.7% Total Return.  The increase in the Fed funds rate is a benefit to banks. PGF invests in Banks by buying their preferred stock. This means that the share price of PGF will continue to increase and its ability to pay the, nearly, 6% per year dividend will also improve. The value of a share depends on the value of the company, which involves a lot more than the value of its assets -- it requires making decisions about what you think will happen to the company in the future. That's inherently not something that can be reduced to a single formula, at least not unless you can figure out how to represent your guesses and your confidence in them in the formula ... and even if you could do all that it would only say what you think the stock is worth; others will be using different numbers and legitimately get different results. Disagreement over value is what the stock market is all about, I'm afraid. The simple answer: The opening price is the price of the first trade of the day and the closing price is the price of the last trade of the day.  And since the stock price change from trade to trade they are usually different. In my opinion, the average investor should not be buying individual stocks. One reason why is that the average investor is not capable of reading financial statements and evaluating whether a stock is overpriced or underpriced. As such, they're often tempted to make buy/sell decisions based solely on the current value of a stock as compared to the price at which they bought it. The real reasons to buy (or sell) a stock is the expectation of future growth of the company (or continued profit and expected dividends). If you aren't able to analyze a company's financial statements and business plan, then you really aren't in a position to evaluate that company's stock price. So instead of asking whether to sell based on a recent drop in stock price, you should be investigating why the stock price is falling, and deciding whether those reasons indicate a trend that you expect to continue.  If you buy and sell stocks based solely on recent trends in the stock price, you probably will end up buying stocks that have recently risen and selling stocks that have recently fallen. In that case, you are buying high and selling low, which is a recipe for poor financial outcomes. It's been awhile since my last finance course, so school me here:  What is the market cap of a company actually supposed to represent? I get that it's the stock price X the # of shares, but what is that actually representing? Revenues? PV of all future revenues? PV of future cash flows?  In any case, good write up. Valuation of tech stocks is quite the gambit, and you've done a good job of dissecting it for a layman.