I haven't read through the actual methodology, but JC Bradbury found this a few years ago: "I looked at how pitchers performed with and without Duncan, controlling for factors such as age, parks, and pitcher quality. I found that Dave Duncan’s pitchers improved their ERAs by about 0.35 runs—yeah, he’s pretty darn good. " I'm not sure what he did to account for defense though, and you've obviously got to take these things with a grain of salt if you haven't read the actual studies. I do think there might actually be something to this "Dave Duncan Effect".