The American historian, Richard Bulliet, in a work based on the quantitative use of the onomastic data as furnished by scholarly biographical dictionaries, concluded that it was only in the 10th century when the Andalusi emirate was firmly established and developed into the greatest power of the western Mediterranean under Caliph Abd ar-Rahman III, that the numerical ratio of Muslims and Christians in Al-Andalus was reversed in favour of the former. Prior to the middle of this century, he asserts, the population of Al-Andalus was still half Christian.