Statistics for Interaction between physical and psychosocial work risk factors for low back symptoms : a study of prevalence, risk factors, and interaction between physical and psychosocial work risk factors for low back symptoms and its consequences (reduced activities and absenteeism) in a random sample of workers in New Zealand and in Indonesian coal mining workers : a thesis presented in fulfilment of the requirements for the degree of Doctor of Philosophy in Ergonomics at Massey University, Manawatu, New Zealand