code: def _get_site_amplification(self, sites, C):
          
          ssa, ssb, ssc, ssd, sse = self._get_site_type_dummy_variables(sites)
  
          return (C['sA'] * ssa) + (C['sB'] * ssb) + (C['sC'] * ssc) + \
              (C['sD'] * ssd) + (C['sE'] * sse)
docs: Compute the fourth term of the equation 1 described on paragraph :
          The functional form Fs in Eq. (1) represents the site amplification and
          it is given by FS = sj Cj , for j = 1,...,5, where sj are the
          coefficients to be determined through the regression analysis,
          while Cj are dummy variables used to denote the five different EC8
          site classes