Correlation between patients' reasons for encounters/health problems and population density in Japan: a systematic review of observational studies coded by the International Classification of Health Problems in Primary Care (ICHPPC) and the International Classification of Primary care (ICPC). - Makoto Kaneko; Ryuichi Ohta; Naoki Nago; Motoharu Fukushi; Masato Matsushima (BMC Family Practice, 2017)