At my first firm, I started in a group of about a dozen guys. A year later, I was the only one still trading and it wasn't at that firm. At the one I'm at now, out of the group of like 25, in about 2 years, 2 got fired, 1 left for a better deal at another firm and 2 left because they got really good job offers from their old jobs, so the other 20 are still with them trading at various levels. Each office is different.
