lirat                  package:VGAM                  R Documentation

_L_o_w-_i_r_o_n _R_a_t _T_e_r_a_t_o_l_o_g_y _D_a_t_a

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

     Low-iron rat teratology data.

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

     data(lirat)

_F_o_r_m_a_t:

     A data frame with 58 observations on the following 4 variables.

     '_N' Litter size.

     '_R' Number of dead fetuses.

     '_h_b' Hemoglobin level.

     '_g_r_p' Group number.  Group 1 is the untreated (low-iron) group,
          group 2 received injections on day 7 or day 10 only,  group 3
          received injections on days 0 and 7, and group 4 received
          injections weekly.

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

     The following description comes from Moore and Tsiatis (1991). 
     The data comes from the experimental setup from Shepard et al.
     (1980), which is typical of studies of the effects of chemical
     agents or dietary regimens on fetal development in laboratory
     rats.

     Female rats were put in iron-deficient diets and divided into 4
     groups. One group of controls was given weekly injections of iron
     supplement to bring their iron intake to normal levels, while
     another group was given only placebo injections.  Two other groups
     were given fewer iron-supplement injections than the controls. 
     The rats were made pregnant, sacrificed 3 weeks later, and the
     total number of fetuses and the number of dead fetuses in each
     litter were counted.

     For each litter the number of dead fetuses may be considered to be
     Binomial(N,p) where N is the litter size and p is the probability
     of a fetus dying. The parameter p is expected to vary from litter
     to litter, therefore the total variance of the proportions will be
     greater than that predicted by a binomial model, even when the
     covariates for hemoglobin level and experimental group are
     accounted for.

_S_o_u_r_c_e:

     Moore, D. F. and Tsiatis, A. (1991) Robust Estimation of the
     Variance in Moment Methods for Extra-binomial and Extra-Poisson
     Variation. _Biometrics_, *47*, 383-401.

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

     Shepard, T. H., Mackler, B. and Finch, C. A. (1980) Reproductive
     studies in the iron-deficient rat. _Teratology_, *22*, 329-334.

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

     ## Not run: 
     data(lirat)
     attach(lirat)
     # cf. Figure 3 of Moore and Tsiatis (1991)
     plot(hb, R/N, pch=as.character(grp), col=grp, las=1,
          xlab="Hemoglobin level", ylab="Proportion Dead")
     detach(lirat)
     ## End(Not run)

