Meriwether's group had been imitated even in its days at Salomon Brothers. In the market for mortgage-backed securities, a crucial issue, as noted above, is calculating the impact of homeowners' "pre-payment" option. The calculation was a non-trivial modeling task that typically took the form of adjusting the "spread" of the yield of mortgage-backed bonds over the yield of Treasury bonds of similar maturities to take the pre-payment option into account. … "The people making more money," he says, "were the ones with the better models, the Meriwethers of the world." Those who were less sophisticated in their modeling learned from the Salomon group by what Roll calls "mimicry": by inferring from Salomon's trading the features its model must have. Roll puts it this way: "If you saw Meriwether going long [that is, buying a mortgage-backed bond] with an option-adjusted spread you thought was five basis points, you knew that his model said it's 100 basis points." In consequence, less experienced participants in the market would ask themselves what they would have to do to their pre-payment model to generate a larger spread, saying to themselves, in Roll's words, "Let's tinker with [the model] and see if we can get that."

Donald MacKenzie, An Engine, Not a Camera, loc. 1023