Position Sizing is the part of the trading system that determines how large a position you will put on throughout the course of a trade
Professional gamblers have long claimed that there are two basic position-sizing strategies - martingale and anti-martingale. Martingale strategies increase one’s bet size when equity decreases (during a losing streak). Anti-martingale strategies, on the other hand, increase one’s bet size during winning streaks or when one’s equity increases.
If you’ve ever played roulette or craps, the purest form of martingale strategy might have occurred to you. It simply amounts to doubling your bet size when you lose. For example, if you lose $1, you bet $2. If you lose $2 then you bet $4. If you lose $4, you bet $8. When you finally win, which you will eventually do, you will be ahead by your original bet size.
Casinos love people who play such martingale strategies. First, any game of chance will have losing streaks. And when the probability of winning is less than 50 percent, the losing streaks could be quite significant. Let’s assume that you have a streak of 10 consecutive losses. If you had started betting $1, then you will have lost $2,047 over the streak. You will now be betting $2048 to get your original dollar back. Thus, your win-loss ratio at this point - for less than a 50:50 bet - is 1 to 4,095. You will be risking over $4,000 to get $1 in profits. And to make matters worst, since some people might have unlimited bankrolls, the casinos have betting limits. At a table that allows a minimum bet of $1, you probably couldn’t risk more than $100. As a result, martingale betting strategies generally do not work - in the casinos or in the market.
If your risk continues to increase during a losing streak, you will eventually have abig enough streak to cause you to go bankrupt. And even if your bankroll was unlimited, you would be commiting yourself to risk-to-reward strategies that no human being could tolerate psychologically.
Anti-martingale strategies, which call for larger risk during a winning streak, do work - both in gambling arena and in the investment arena. Smart gamblers know to increase their bets, within certain limits, when they are winning. And the same is true for trading or investing. Position-sizing systems that work call for you to increase your position size when you make money. That holds for gambling, for trading, and for investing.
The purpose of postion-sizing is to tell you how many units (shares or contracts) you going to put on, given the size of your account. For example, a position-sizing decision might be that you don’t have enough money to put on any positions because the risk is too big. It allows you to determine your reward and risk characteristics by determining how many units you will risk on a given trade and in each trade in a portfolio. It also helps you equalize you trade exposure in the elements in your portfolio.
Some people believe they are “doing an adequate job of position sizing” by having a “money management stop.” Such a stop would be one in which you get out of your position when you lose a predetermined amount of money - say $1,000. However, this kind of stop does not tell you “how much” or “how many,” so it really as nothing to do with position sizing. Controlling risk by determining the amount of loss if you are stopped out is not the same as controlling risk through a position-sizing model that determines “how many” or if you can even afford to hold one position at all.
