zero                  package:VGAM                  R Documentation

_T_h_e _z_e_r_o _A_r_g_u_m_e_n_t _i_n _V_G_A_M _F_a_m_i_l_y _F_u_n_c_t_i_o_n_s

_D_e_s_c_r_i_p_t_i_o_n:

     The 'zero' argument allows users to conveniently model certain
     linear/additive predictors as intercepts only.

_D_e_t_a_i_l_s:

     Often a certain parameter needs to be modelled simply while other
     parameters in the model may be more complex, for example, the
     lambda parameter in LMS-Box-Cox quantile regression should be
     modelled more simply compared to its mu parameter. Another example
     is the xi parameter in a GEV distribution which is should be
     modelled simpler than its mu parameter. Using the 'zero' argument
     allows this to be fitted conveniently without having to input all
     the constraint matrices explicitly.

     The 'zero' argument should be assigned an integer vector from the
     set {'1:M'} where 'M' is the number of linear/additive predictors.
      Full details about constraint matrices can be found in the
     references.

_V_a_l_u_e:

     Nothing is returned. It is simply a convenient argument for
     constraining certain linear/additive predictors to be an intercept
     only.

_W_a_r_n_i_n_g:

     The use of other arguments may conflict with the 'zero' argument.
     For example, using 'constraints' to input constraint matrices may
     conflict with the 'zero' argument. Another example is the argument
     'parallel'. In general users should not assume any particular
     order of precedence when there is potential conflict of
     definition. Currently no checking for consistency is made. 

     The argument 'zero' may be renamed in the future to something
     better.

_S_i_d_e _E_f_f_e_c_t_s:

     The argument creates the appropriate constraint matrices
     internally.

_N_o_t_e:

     In all 'VGAM' family functions 'zero=NULL' means none of the
     linear/additive predictors are modelled as intercepts-only. Almost
     all 'VGAM' family function have 'zero=NULL' as the default, but
     there are some exceptions, e.g., 'binom2.or'.

     Typing something like 'coef(fit, matrix=TRUE)' is a useful way to
     ensure that the 'zero' argument has worked as expected.

_A_u_t_h_o_r(_s):

     T. W. Yee

_R_e_f_e_r_e_n_c_e_s:

     Yee, T. W. and Wild, C. J. (1996) Vector generalized additive
     models. _Journal of the Royal Statistical Society, Series B,
     Methodological_, *58*, 481-493.

     Yee, T. W. and Hastie, T. J. (2003) Reduced-rank vector
     generalized linear models. _Statistical Modelling_, *3*, 15-41.

     <URL: http://www.stat.auckland.ac.nz/~yee>

_S_e_e _A_l_s_o:

     'constraints'.

_E_x_a_m_p_l_e_s:

     args(multinomial)
     args(binom2.or)
     args(gpd)

     #LMS quantile regression example
     data(bminz)
     fit = vglm(BMI ~ bs(age, df=4), fam=lms.bcg(zero=c(1,3)),
                data=bminz, trace=TRUE)
     coef(fit, matrix=TRUE)

