Resonance signals in pion-nucleon scattering from speed plots and time   delay
  We compare a number of methods used to locate resonances. These include the speed plot, the time-delay method of Eisenbud and Wigner, the time-delay matrix of Smith, and a modification by Ohmura. Numerical results show a consistency not previously reported. One time delay method gives the most consistent results, supporting the conclusions of a recent theoretical study. 
R.L.Workman and R.A.Arndt