Can a stop loss order be triggered by random price?
Typically this isn't a random order- having a small volume just means it's not showing on the chart, but it is a vlid price point. Same thing would've happened if it would've been a very large order that shows on the chart. Consider also that this could have been the first one of many transactions that go far below your stop point - would you not have wanted it to be executed then, at this time, as it did? Would you expect the system to look into future and decide that this is a one time dip, and not sell; versus it is a crash, and sell? Either way, the system cannot look in the future, so it has no way to know if a crash is coming, or if it was a short dip; therefore the instrcutions are executed as given - sell if any transfer happens below the limit. To avoid that (or at least reduce the chance for it), you can either leave more distance (and risk a higher loss when it crashes), or trade higher volumes, so the short small dip won't execute your order; also, very liquid stocks will not show such small transaction dips.
The strategy could conceivably work if you had sufficient quantity of shares to fill all of the outstanding buy orders and fill your lower buy orders.  But in this case you are forcing the market down by selling and reinforcing the notion that there is a sell off by filling ever lower buy orders.  There is the potential to trigger some stop loss orders if you can pressure it low enough.  There is a lot of risk here that someone sees what you are doing and decides to jump in and buy forcing the price back up.  Could this work sure.  But it is very risky and if you fail to create the panic selling then you risk losing big.  I also suspect that this would violate SEC Rules and several laws.  And if the price drops too far then trading on the stock would be halted and is likely to return at the appropriate price. Bottom line I can not see a scenario where you do not trigger the stop, net a profit and end up with as many or more stock that you had in the first place. "Of the two, an option is a more reliable but more expensive means to get rid of a stock. As sdg said, a put option is basically an insurance policy on the stock; you pay a certain price for the contract itself, which locks in a sale price up to a particular future date. If the stock depreciates significantly, you exercise the option and get the contract price; otherwise you let the contract expire and keep the stock. Long-term, these are bad bets as each expired contract will offset earnings, but if you foresee a near-term steep drop in the stock price but aren't quite sure, a put option is good peace of mind. A sell stop order is generally cheaper, but less reliable. You set a trigger price, say a loss of 10% of the stock's current value. If that threshold is reached, the stop order becomes a sell order and the broker will sell the stock on the market, take his commission (or a fixed price depending on your broker) and you get the rest. However, there has to be a buyer willing to buy at that price at the moment the trigger fires; if a stock has lost 10% rapidly, it's probably on the way down hard, and the order might not complete until you realize a 12% loss, or a 15%, or even 20%. A sell stop limit (a combination stop order and limit order) allows you to say that you want to sell if the stock drops to $X, but not sell if it drops below $X-Y. This allows you to limit realized losses by determining a band within which it should be sold, and not to sell above or below that price. These are cheaper because you only pay for the order if it is executed successfully; if you never need it, it's free (or very cheap; some brokers will charge a token service fee to maintain a stop or stop limit). However, if the price drops very quickly or you specify too narrow a band, the stock can drop through that band too quickly to execute the sell order and you end up with a severely depreciated stock and an unexercised order. This can happen if the company whose stock you own buys another company; VERY quickly, both stocks will adjust, the buying company will often plummet inside a few seconds after news of the merger is announced, based on the steep drop in working capital and/or the infusion of a large amount of new stock in the buying company to cover the equity of the purchased company. You end up with devalued stock and a worthless option (but one company buying another is not usually reason to sell; if the purchase is a good idea, their stock will recover). Another option which may be useful to you is a swaption; this basically amounts to buying a put option on one financial instrument and a call on another, rolled into one option contract specifying a swap. This allows you to pick something you think would rise if your stock fell and exchange your stock for it at your option. For example, say the stock on which you buy this swaption is an airline stock, and you contract the option to swap for oil. If oil surges, the airline's stock will tank sharply, and you win both ways (avoiding loss and realizing a gain). You'd also win if either half of this option realized a gain over the option price; oil could surge or the airline could tank and you could win. You could even do this ""naked"" since its your option; if the airline's stock tanks, you buy it at the crashed price to exercise the option and then do so. The downside is a higher option cost; the seller will be no fool, so if your position appears to be likely, anyone who'd bet against you by selling you this option will want a pretty high return." So far the answer is: observe the general direction of the market, using special tools if needed or you have them available (.e.g. Bollinger bands to help you understand the current trend) at the right time per above, do the roll with stop loss in place (meaning roll at a pre-determined max loss), and also a trailing stop loss if the roll works in your favor, to capture the profits on the roll. This trade was a learning experience.  I sold the option at $20 thinking I'd get back in later in the day with the further out option at a good price, as the market goes back and forth.  The underlying went up and never came back.  I finally gritted my teeth and bought the new option at 23.10 (when it would have cost me about 20.20 before), i.e. a miss/loss of $3 on $20. The underlying continued to rise, from that point (hasn't been back), and now the option price is $29. Of course one needs to make sure the Implied Volatility of the option being left and the option going to is good/fair, and if not, either roll further out in time, nearer in time, our up / down the strike prices, to find the right target option.  After doing that, one might do the strategy above, i.e. any good trade mgmt type strategy:  seek to make a good decision, acknowledge when you were wrong (with stop loss), and act. Or, if you're right, cash in smartly (i.e. trailing stops). 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. "Firstly, if a stock costs $50 this second, the bid/ask would have to be 49/50.  If the bid/ask were 49/51, the stock would cost $51 this second.  What you're likely referring to is the last trade, not the cost.  The last trading price is history and doesn't apply to future transactions. To make it simple, let's define a simple order book.  Say there is a bid to buy 100 at $49, 200 at $48, 500 at $47.  If you place a market order to sell 100 shares, it should all get filled at $49.  If you had placed a market order to sell 200 shares instead, half should get filled at $49 and half at $48.  This is, of course, assuming no one else places an order before you get yours submitted.  If someone beats you to the 100 share lot, then your order could get filled at lower than what you thought you'd get.  If your internet connection is slow or there is a lot of latency in the data from the exchange, then things like this could happen.  Also, there are many ECNs in addition to the exchanges which may have different order books.  There are also trades which, for some reason, get delayed and show up later in the ""time and sales"" window. But to answer the question of why someone would want to sell low... the only reason I could think is they desire to drive the price down." "No one is quite sure what happened (yet).  Speculation includes: The interesting thing is that Procter & Gamble stock got hammered, as did Accenture.  Both of which are fairly stable companies, that didn't make any major announcements, and aren't really connected to the current financial instability in Greece.  So, there is no reason for there stock prices to have gone crazy like that.  This points to some kind of screw up, and not a regular market force. Apparently, the trades involved in this event are going to be canceled. Edit #1: One thing that can contribute to an event like this is automatic selling triggered by stop loss orders.  Say someone at Citi makes a mistake and sells too much of a stock.  That drives the stock price below a certain threshold.  Computers that were pre-programmed to sell at that point start doing their job.  Now the price goes even lower.  More stop-loss orders get triggered.  Things start to snowball.  Since it's all done by computer these days something like this can happen in seconds.  All the humans are left scratching their heads. (No idea if that's what actually happened.) Edit #2: IEEE Spectrum has a pretty concise article on the topic.  It also includes some links to follow. Edit #3 (05/14/2010): Reuters is now reporting that a trader at Waddell & Reed triggered all of this, but not through any wrongdoing. Edit #4 (05/18/2010): Waddell & Reed claims they didn't do it.  The House Financial Services Subcommittee investigated, but they couldn't find a ""smoking gun"".  I think at this point, people have pretty much given up trying to figure out what happened. Edit #5 (07/14/2010): The SEC still has no idea.  I'm giving up. :-)" Capping the upside while playing with unlimited downside is a less disciplined investment strategy vis-a-vis a stop-loss driven strategy. Whether it is less risky or high risky also depends on the fluctuations of the stock and not just long-term movements. For example, your stop losses might get triggered because of a momentary sharp decline in stock price due to a large volume transaction (esp more so in small-cap stocks). Although, the stock price might recover from the sudden price drop pretty soon causing a seemingly preventable loss. That being said, playing with stop losses is always considered a safer strategy. It may not increase your profits but can certainly cap your losses. "In US public stock markets there is no difference between the actions individual retail traders are ""permitted"" to take and the actions institutional/corporate traders are ""permitted"" to take.  The only difference is the cost of those actions.  For example, if you become a Registered Market Maker on, say, the BATS stock exchange, you'll get some amazing rebates and reduced transaction prices; however, in order to qualify for Registered Market Maker status you have to maintain constant orders in the book for hundreds of equities at significant volumes.  An individual retail trader is certainly permitted to do that, but it's probably too expensive. Algorithmic trading is not the same as automated trading (algorithmic trading can be non-automated, and automated trading can be non-algorithmic), and both can be anywhere from low- to high-frequency.  A low-frequency automated strategy is essentially indistinguishable from a person clicking their mouse several times per day, so: no, from a legal or regulatory perspective there is no special procedure an individual retail trader has to follow before s/he can automate a trading strategy.  (Your broker, on the other hand, may have all sorts of hoops for you to jump through in order to use their automation platform.) Last (but certainly not least) you will almost certainly lose money hand over fist attempting bid-ask scalping as an individual retail trader, whether your approach is algorithmic or not, automated or not.  Why?  Because the only way to succeed at bid-ask scalping is to (a) always be at/near the front of the queue when a price change occurs in your favor, and (b) always cancel your resting orders before they are executed when a price change occurs against you.  Unless your algorithms are smarter than every other algorithm in the industry, an individual retail trader operating through a broker's trading platform cannot react quickly enough to succeed at either of those.  You would have to eschew the broker and buy direct market access to even have a chance, and that's the point at which you're no longer a retail trader. Good luck!"