untangle.specials          package:survival          R Documentation

_H_e_l_p _P_r_o_c_e_s_s _t_h_e '_s_p_e_c_i_a_l_s' _A_r_g_u_m_e_n_t _o_f _t_h_e '_t_e_r_m_s' _F_u_n_c_t_i_o_n.

_D_e_s_c_r_i_p_t_i_o_n:

     Given a 'terms' structure and a desired special name, this returns
     an index appropriate for subscripting the 'terms' structure and
     another appropriate for the data frame.

_U_s_a_g_e:

     untangle.specials(tt, special, order=1)

_A_r_g_u_m_e_n_t_s:

      tt: a 'terms' object. 

 special: the name of a special function, presumably used in the terms
          object. 

   order: the order of the desired terms.  If set to 2, interactions
          with the special function will be included. 

_V_a_l_u_e:

     a list with two components: 

    vars: a vector of variable names, as would be found in the data
          frame, of the specials. 

   terms: a numeric vector, suitable for subscripting the terms
          structure, that indexes the terms in the expanded model
          formula which involve the special. 

_E_x_a_m_p_l_e_s:

     formula<-Surv(tt,ss)~x+z*strata(id)
     tms<-terms(formula,specials="strata")
     ## the specials attribute
     attr(tms,"specials")
     ## main effects 
     untangle.specials(tms,"strata")
     ## and interactions
     untangle.specials(tms,"strata",order=1:2)

