This paper reports analysis of 2006-2007 on-entry assessment data from the Performance Indicators in Primary Schools Baseline Assessment (PIPS-BLA) of random samples of students in England, Scotland, New Zealand and Australia. The analysis aimed, first, to investigate the validity and reliability of that instrument across countries and sexes, and, second, to investigate what students know and can do the year before they start formal schooling. Data was analysed using the Rasch measurement model and interactive software (RUMM2020). Results indicated that three scales, Reading, Vocabulary and Mathematics, represent (with the exception of very few items) single variables which have the same meaning across all four countries and both sexes. The three scales may also be combined and regarded as a single measure at a higher level of scale. Developmental trends for children starting school were similar for all four countries. (Contains 12 figures and 5 tables.)