Casualty reports are categorized into two types: one for leave of
absence for less than four days and the other for four days or more. In our study, we studied 34,195 samples
randomly selected from the casualty reports for leave of absence for four days or more in 2006; the data were
provided by the Ministry of Health, Labor and Welfare. Since the total number of reports in that year was 134,298,
the extraction rate of the subject data was about 25.5%.