2. Materials and methods
2.1. Data
Data was extracted from an existing database (Meiji University, Kawasaki, Japan). The database was constructed in the following manner. Approximately 140 commercial farms using a farm management program PigCHAMP® in Japan were requested from December 2002 to mail their data files to the university, when they purchased the software or renewed their yearly maintenance contract. By August 31, 2003, the data files were received from 119 breeding farms. Measurements of serviced females for 2002 were extracted from each data file and used in this study.