
Games of chance are influenced by probability. If you flip a coin 49 times and it comes up heads, there is a 50-50 chance that the coin will come up heads on the 50th flip. That is the essence of probability theory. Past history does not influence future results. Everything seems to fall neatly on a bell-shape curve (also known as a Gaussian, or normal distribution).
I am not a mathematician, but I do know that there is a 68% probability that prices will end within one standard deviation from the mean over a given period of time. There is a 95% chance that it will end within two standard deviations. All pretty basic stuff — assuming you know what a standard deviation is.
A number of brokers offer clients a tool called a Probability Calculator. The tool is very fun to play with, in my opinion. And it is REALLY useful if you are evaluating the probability of success on a spread option trade. With a credit spread, for instance, you are collecting premium up front. Usually you have at least one break-even point. If you are trading an iron condor or a butterfly strategy you may have two break-even points. Based upon an option pricing model, the probability calculator can tell you the calculated probability of your trade being successful. Success is defined as making one penny, or more, on the trade.
To use the calculator, simply enter the underlying symbol, and establish your target prices and your time frame. If you have that iron condor trade on your mind, enter the break-even points on the strategy as your price targets. The calculator will then instantly give you the probability of touching your target prices within your defined time frame a well as the probability of your strategy finishing above, or below, your target prices — assuming you take it to expiration. Everything is a risk-return trade-off, but if you are doing a credit trade, you want a better than 50% chance of having the trade become successful.
I use a probability calculator to check just about any spread trade I want to put on. First, I establish my strategy in the more powerful, and more graphic, Profit and Loss Calculator. Here’s the rub, though. (Very important.) You have to believe that probability theory is the best theory to explain price results in stock markets. That’s been questioned lately as we are seeing more and more black swan events that are supposed to happen only once every 500 hundred years.
You can find a Probability Calculator at TradeKing Zecco or OptionsHouse. But before you dive in, read this review of a book by Benoit Mandelbrot’s called “The Misbehavior of Markets” along with the comments on the review. The basic idea is that probability theory is flawed with it comes to predicting stock prices.
It may get you thinking that perhaps probability theory is not all it is cracked up to be.



My buddy, Steve Claussen, from OptionsHouse had a good example of a “married put” on