mohsen rezaeian department of epidemiology and biostatistics, occupational environmental research center, rafsanjan medical school, rafsanjan university of medical sciences, rafsanjan, ir iran; department of epidemiology and biostatistics, occupational environmental research center, rafsanjan medical school, rafsanjan university of medical sciences, rafsanjan, ir iran. tel: +98-3434331315, fax: +98-3431315003