NEW DELHI: India started counting its billion-plus population on Thursday for a new census that will gather biometric data for the first time from across the vast and chaotic nation. The census, undertaken every decade since 1872, will see 2.5 million staff fan out over the country. As well as counting and classifying the world’s second biggest population, census officials will collect biometric data on every person aged over 15 for a new National Population Register. Every resident will be fingerprinted and photographed — a process termed by Home Minister P Chidambaram as “the biggest exercise... since humankind existed.” Personal details will also be recorded, such as declared nationality and marital status, as well as statistics on the proportion of bank account holders, cellphone owners and Internet users.