<b>constant position size?</b>Originally posted by Eldredge
If trader 1 buys $100,000 worth of stock each time, he will net $5,000.
If trader 2 buys $400,000 worth of stock each time, he will net $20,000.
There are lots of different ways to look at using margin, and money management is always very critical.
Yes, its true that maintaining a constant position size would have the effect that you state, but only if the 15% gain trade occurs BEFORE the 10% loss trade.
In the event that the 10% loss occurs first, Trader 1 would have only 90k in their account after the 10% losing trade (and 103.5k after the 15% up trade). In the event that the 10% loss occurs first for the margin user, Trader 2 would have only 60k in their account after the 10% losing trade and could only create a 240k position on the second trade (they would again have 96k after the 15% up trade, if they committed full buying power to the second trade after the first losing trade). Constant position size trading only works if you use a fraction of the maximum acceptable buying power (or you are very lucky to have your wins come before your losses).
<b>margin modulates risk</b>
I agree with CalTrader's perspective that margin is a risk parameter that is gives some control over risk (and that one generally wants to minimize). I think (correct me if I'm wrong) that Eldredge's counterexample illustrates the interaction of margin as part of a money management policy. Choosing to use margin is just a position sizing decision that can work well if the wins outweigh the losses by a wide enough difference. Its very tricky stuff (mathematically, you can show that there is an optimum amount of margin for any covariance matrix of expected trading returns and that that optimum margin level can be greater than 1 in some cases).
In any case, as tntneo points out, margin is especially dangerous in a losing streak -- a very bad loss can blow-up an overleveraged account. Swing-traders that hold overnight positions must be especially careful of margin because a vicious overnight plunge in price can wipe out an overleveraged position (or buoyant news can squeeze an overleveraged short-seller to death). I would also second Rigel's observation that even daytraders can get killed by margin if a trading halt and subsequent price move occurs before they can exit an overleveraged position. Traders (me included) think we have more control over our destinies than we actually do.
Margin means both living on borrowed money AND borrowed time, IMHO. Sometimes its OK to borrow both time and money (but it is risky).
Wishing safe trading to all,
-Traden4Alpha

). Anyhoo, the example also works if you construct a more complex set of trades with multiple stocks that lead to a 10% drawdown and then a 15% gain off that bottom. If you replicate those trades under two money management strategies: a conservative one (using 1:1 margin or cash-only); and an aggressive one (using 4:1 margin) then you will find that trader using margin is worse-off.