A young Salomon Brothers trader named Howie Rubin began to calculate the probability of homeowners' prepaying their mortgages. He discovered that the probability varied according to where they lived, the length of time their loans had been outstanding, and the sizes of their loans. He used historical data collected by Lew Ranieri's research department. The researchers were meant to be used like scientific advisers at an arms talk. More often, however, they were treated like the water boys on the football team. But the best traders knew how to use the researchers well. The American homeowner became, to Rubin and the research department, a sort of laboratory rat. The researchers charted how previously sedentary homeowners jumped and started in response to the shock of changes in the rate of interest. Once a researcher was satisfied that one group of homeowners was more likely than another to behave irrationally, and pay off low-interest-rate mortgages, he would inform Rubin, who then bought their mortgages. The homeowners, of course, never knew that their behavior was so closely monitored by Wall Street.