Running examples in 'gammi-Ex.R' failed
The error most likely occurred in:

> ### Name: exam
> ### Title: Cross-Classified Examination Data
> ### Aliases: exam
> ### Keywords: datasets
> 
> ### ** Examples
> 
> # load 'exam' help file
> ?exam
exam                   package:gammi                   R Documentation

_C_r_o_s_s-_C_l_a_s_s_i_f_i_e_d _E_x_a_m_i_n_a_t_i_o_n _D_a_t_a

_D_e_s_c_r_i_p_t_i_o_n:

     Scores on secondary school leaving examinations (response) and
     verbal reasoning scores in primary school (fixed effect) for 3435
     students in Fife, Scotland. The students are cross-classified in
     148 primary schools (random effect) and 19 secondary schools
     (random effect).

_U_s_a_g_e:

     data("exam")
     
_F_o_r_m_a_t:

     A data frame with 3435 observations on the following 4 variables.

     'VRQ.score' Verbal Reasoning Quotient obtained in primary school
          (integer vector ranging from 70 to 140)

     'Exam.score' Leaving examination score obtained in secondary
          school (integer vector ranging from 1 to 10)

     'Primary.school' Primary school identifier (factor with 148
          levels)

     'Secondary.school' Secondary school identifier (factor with 19
          levels)

_D_e_t_a_i_l_s:

     The VRQ scores were obtained at age 12 (right before entering
     secondary school), and the Exam scores were obtained at age 16
     (right before leaving secondary school). The VRQ scores are
     constructed to have a population mean of 100 and population
     standard deviation of 15. The goal is to predict the leaving Exam
     scores from the VRQ scores while accounting for the primary and
     secondary school cross-classifications.

_S_o_u_r_c_e:

     Data Obtainable from:
     <https://www.bristol.ac.uk/cmm/team/hg/msm-3rd-ed/datasets.html>

_R_e_f_e_r_e_n_c_e_s:

     Goldstein, H. (2011). Multilevel Statistical Models, 4th Edition.
     Chapter 12: Cross-classified data structures (pages 243-254).
     doi:10.1002/9780470973394 <https://doi.org/10.1002/9780470973394>

     Paterson, L. (1991). Socio-economic status and educational
     attainment: a multidimensional and multilevel study. Evaluation
     and Research in Education, 5, 97-121.
     doi:10.1080/09500799109533303
     <https://doi.org/10.1080/09500799109533303>

_E_x_a_m_p_l_e_s:

     # load 'exam' help file
     ?exam
     
     # load data
     data(exam)
     
     # header of data
     head(exam)
     
     # fit model
     mod <- gammi(Exam.score ~ VRQ.score, data = exam,
                  random = ~ (1 | Primary.school) + (1 | Secondary.school))
                  
     # plot results
     plot(mod)
     
     # summarize results
     summary(mod)
     
     # variance parameters
     mod$VarCorr
     
> 
> # load data
> data(exam)
> 
> # header of data
> head(exam)
  VRQ.score Exam.score Primary.school Secondary.school
1       111         10              1                9
2       100          3              1                9
3        86          2              1                9
4        94          3              1                9
5        70          2              1                9
6        83          2              1                9
> 
> # fit model
> mod <- gammi(Exam.score ~ VRQ.score, data = exam,
+              random = ~ (1 | Primary.school) + (1 | Secondary.school))
Error in crossprod(x, y) : 
  requires numeric/complex matrix/vector arguments
Calls: gammi ... gammi.formula -> gammi.default -> rbind -> crossprod
Execution halted