Obviously, if you make a mistake and it turns into a winner, you'll count that too. It's a positive R value, rather than a negative one. But these are the most deadly because they make you think it's okay to make a mistake. And even when you count winners in your mistakes, I still think you'll find that the average mistake is worth 2-4R against you.

It's a little like poker. Let's say I'm betting with an AK in my hand and an AK on the board. Someone is better against me with pocket sixes. (This actually happened). I raise their bets and they still call. Their bet is absolute ridiculous. First, they might think I'm bluffing, but if they paid any attention to me they'd know I'm a conserative player and I usually have something if I'm betting. Thus, they have to know they are behind in the hand.
Secondly, there are only two cards left in the deck (two sixes) that will beat me. They have a about a one in 20 chance of beating me but they are still calling my bets. Really dumb. But what happens. On the river another six comes up and they win a huge pot. I'm a little upset because an idiot with a lot of luck beat me. But they are reinforced to do the same thing over and over again to win a big pot. However, THEY WILL LOSE 19 out of 20 times.

That's what happens in the markets when you make a mistake and you make money. You are reinforced to do it again. And that's why its so important to keep track of your mistakes.

As for the person who said this is not a useful exercize.... we'll my job as a coach is to teach people what works. Typically, people who don't do what the coach says are kicked off the team. However, free coaching, when you are not on a team, usually isn't valued. I once told a firm that if they kept doing what they were doing they'd go bankrupt. They disagreed and went under ($11 million lost) in about six months. Perhaps if they'd paid me a million for the adivce they'd still be in business.