Why do more floating shares mean less volatility for the stock?
More shares mean less volatility because it takes a larger number of trades, a larger number of shares per trade, or a combination of both to raise or lower the stock price. Institutional investors (mutual funds, pensions, hedge funds, other investment firms, etc) are the sorts of organizations with the large amounts of money needed to move a stock price one way or the other.  But the more floating shares there are in a company, the harder it is for one or two firms to move a stock price.  A company with fewer floating shares wouldn't require as many trades (or as many shares per trade) to see wider swings in price. When it comes to stock price, insider trading isn't the same as manipulation.  In the (surprisingly few) cases of insider trading that are prosecuted, it tends to be an individual (or small group) with early access to information that the broader market doesn't have being able to buy or sell ahead of the broader market.  Their individual sales are seldom if ever enough to noticeably move a stock price.  They're locking in profit or limiting a loss.  Manipulation might (but doesn't always) precede insider trading, if misinformation (or truth) is released for the purpose of creating a situation that can be profited from via a trade or trades.
"I looked at data from Sept 2010 to present: Standard deviation is what shows the spread shape of returns over time, and it meanS that about 2/3 of the time, AAPL return was within +/- 1.65 higher/lower than the daily average return which was .21 %. Not sure where to go with this except to suggest that in fact, AAPL is more volatile than the S&P and even another random tech company. With time, I'd probably come up with a list of stock more volatile. I know that when I look at a list of stocks I track on Yahoo, there are always a few that are just as volatile on a given day.  Excel makes the above analysis easy to do for a given stock, and it's actually an interesting exercise, at least for me. Disclaimer - the shape of stock returns is not a bell curve, and STdev is just a best fit.  Edit - given more time to tinker on excel, it would be interesting to see how the stock's volatility tracked over the years, did it increase or does it feel that way due to the high price? A $20 swing on a $600 stock is the same as a $2 swing on a $60 stock, yet ""up $20"" sounds huge." "So, if an out-of-the-money option (all time value) has a price P (say $3.00), and there are N days... The extrinsic value isn't solely determined by time value as your quote suggests. It's also based on volatility and demand. Here is a quote from http://www.tradingmarkets.com/options/trading-lessons/the-mystery-of-option-extrinsic-value-767484.html distinguishing between extrinsic time value and extrinsic non-time value: The time value of an option is entirely predictable. Time value premium declines at an accelerating rate, with most time decay occurring in the last one to two months before expiration. This occurs on a predictable curve. Intrinsic value is also predictable and easily followed. It is worth one point for every point the option is in the money. For example, a call with a strike of 30 has three points of intrinsic value when the current value of the underlying stock is $33 per share; and a 40 put has two points of intrinsic value when the underlying stock is worth $38. The third type of premium, extrinsic value, increases or decreases when the underlying stock changes and when the distance between current value of stock and strike of the option get closer together. As a symptom of volatility, extrinsic value may be greater for highly volatile underlying stock, and lower for less volatile stocks. Extrinsic value is the only classification of option premium that is unpredictable. The SPYs you point out probably had a volatility component affecting value. This portion is a factor of expectations or uncertainty. So an event expected to conclude prior to expiration, but of unknown outcome can cause theta to be higher than p/n. For example, a drug company is being sued and the outcome of a trial will determine whether that company pays out millions or not. The extrinsic will be higher than p/n prior to the outcome of the trial then drops after.  Of course, the most common situation where this happens is earnings. After the announcement, it's not unusual to see a dramatic drop in the extrinsic portion of options. This is why sometimes a new option trader gets angry when buying calls prior to earnings. When 'surprise' good earnings are announced as hoped, the rise is stock price is largely offset by a fall in extrinsic value giving call holders little or no gain! As for the reverse situation where theta is lower than p/n would expect? Well you can actually have negative theta meaning the extrinsic portion rises over time. (this statement is a little confusing because theta is usually described as negative, but since you describe it as a positive number, negative here means the opposite of what you'd expect).  This is a quote from ""Option Volatility & Pricing"". Keep in mind that they use 'positive' theta to mean the time value increases up over time: Is it ever possible for an option to have a positive theta such that if nothing changes the option will be worth more tomorrow than it is today? When futures options are subject to stock-type settlement, as they currently are in the United States, the carrying cost on a deeply in-the-money option, either a call or a put, can, under some circumstances, be greater than the volatility component. If this happens, and the option is European (no early exercise permitted), it will have a theoretical value less than parity (less than intrinsic value). As expiration approaches, the value of the option will slowly rise to parity. Hence, the option will have a positive theta. Sheldon Natenberg. Option Volatility & Pricing: Advanced Trading Strategies and Techniques (Kindle Locations 1521-1525). Kindle Edition." How I understand it is: supply/demand affect price of stock   negatively/positively, respectively. Correct. Volume is the amount of buying/selling activity in these stocks (more   volume = more fluctuation, right?). Sort of.  Higher volume means higher liquidity.  That is, a stock that is traded more is easier to trade.  It doesn't necessarily mean more fluctuation and in the real world, it often means that these are well-understood stocks with a high amount of analyst coverage.  This tends towards these stocks not being as volatile as smaller stocks with less liquidity. Company revenue (and profit) will help an investor predict company   growth. That is one factor in a stock price.  There are certain stocks that you would buy without them making a profit because their future revenue looks potentially explosive.  However, these stocks are very risky and are bubble-prone. If you're starting out in the share market, it's generally a good idea to invest in index funds (I am not a broker, my advice should not be taken as financial advice).  These funds aggregate risk by holding a lot of different companies.  Also, statistics have shown that over time, buying and holding index funds long term tends to dramatically outperform other investment strategies, particularly for people with low amounts of capital. The stock market, as a whole, is extremely volatile.  During any 3 year period, the market could go up or down.  However, and this is the important point,the market as a whole has historically been a good long term investment. If you need the money in 5 years, then you want to put it in something less volatile (so there's less chance of losing it).  If you need the money in 50 years, put it in the market; the massive growth over those 50 years will more than make up for any short term drops, and you will probably come out ahead. Once you get closer to retirement age, you want to take the money out of stocks and put it in something safer; essentially locking in your profit, and protecting yourself from the possibility of further loss. Something else to consider: everyone lost money in 2008.  There were no safe investments (well, ok, there were a few... but not enough to talk about).  Given that, why would you choose another investment over stocks?  Taking a 50% loss after decades of 10% annual returns is still better than a 50% loss after decades of 5% growth (in fact, after 20 years of growth, it's still 250% better - and that ratio will only improve the longer you leave it in). "The simple answer could be that one or more ""people"" decided to buy. By ""people,"" I don't mean individual buyers of 100 shares like you or me, but typically large institutional investors like Fidelity, who might buy millions of shares at a time. Or if you're talking about a human person, perhaps someone like Warren Buffett. In a ""thinly"" traded small cap stock that typically trades a few hundred shares in a day, an order for ""thousands"" could significantly move the price. This is one situation where more or less ""average"" people could move a single stock." 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. While there are many very good and detailed answers to this question, there is one key term from finance that none of them used and that is Net Present Value. While this is a term generally associate with debt and assets, it also can be applied to the valuation models of a company's share price. The price of the share of a stock in a company represents the Net Present Value of all future cash flows of that company divided by the total number of shares outstanding. This is also the reason behind why the payment of dividends will cause the share price valuation to be less than its valuation if the company did not pay a dividend. That/those future outflows are factored into the NPV calculation, actually performed or implied, and results in a current valuation that is less than it would have been had that capital been retained. Unlike with a fixed income security, or even a variable rate debenture, it is difficult to predict what the future cashflows of a company will be, and how investors chose to value things as intangible as brand recognition, market penetration, and executive competence are often far more subjective that using 10 year libor rates to plug into a present value calculation for a floating rate bond of similar tenor. Opinion enters into the calculus and this is why you end up having a greater degree of price variance than you see in the fixed income markets. You have had situations where companies such as Amazon.com, Google, and Facebook had highly valued shares before they they ever posted a profit. That is because the analysis of the value of their intellectual properties or business models would, overtime provide a future value that was equivalent to their stock price at that time. When volatility is higher, the option is more likely to end up in-the-money. Moreover, when it ends up in-the-money, it is likely to be over the strike price by a greater amount. Consider a call option. With high volatility, moves in the stock price are big - both up moves and down moves. If the stock moves up by a lot, the call option holder will benefit greatly. On the other hand, when the stock moves down, below a certain point the option holder does not care how big a down move the stock has. His downside is limited. Hence, the value of the option is increased by high volatility. I know everyone who searches this is looking for this answer. Bump so people are able to get this concept instead of looking all over the web for it.