boxcox                 package:MASS                 R Documentation

_B_o_x-_C_o_x _T_r_a_n_s_f_o_r_m_a_t_i_o_n_s _f_o_r _L_i_n_e_a_r _M_o_d_e_l_s

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

     Computes and optionally plots profile log-likelihoods for the
     parameter of the Box-Cox simple power transformation `y^lambda'.

_U_s_a_g_e:

     boxcox(object, lambda, plotit, interp, eps, xlab, ylab, ...)

_A_r_g_u_m_e_n_t_s:

  object: a formula or fitted model object.  Currently only `lm' and
          `aov' objects  are handled. 

  lambda: vector of values of `lambda' - default (-2, 2) in steps of
          0.1 

  plotit: logical which controls whether the result should be plotted.
          Default to `true' if a graphics device is currently open. 

  interp: logical which controls whether spline interpolation is used.
          Default to `true' if plotting with `lambda' of length less
          than 100. 

     eps: Tolerance for `lambda = 0'; defaults to 0.02 

    xlab: defaults to `"lambda"' 

    ylab: defaults to `"log-Likelihood"' 

     ...: additional parameters to be used in the model fitting. 

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

     A list of the `lambda' vector and the computed profile
     log-likelihood vector, invisibly if the result is plotted.

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

     If `plotit = TRUE' plots loglik vs `lambda' and indicates a 95%
     confidence interval about the maximum observed value of `lambda'.
     If `interp = TRUE', spline interpolation is used to give a
     smoother plot.

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

     data(trees)
     boxcox(Volume ~ log(Height) + log(Girth), data = trees, 
            lambda = seq(-0.25, 0.25, length = 10))
     data(quine)
     boxcox(Days+1 ~ Eth*Sex*Age*Lrn, data = quine, singular.ok = TRUE,
      lambda = seq(-0.05, 0.45, len = 20))

