<Tr> <Th> Most populous state </Th> <Td> Uttar Pradesh (199,812,341) </Td> </Tr> <Tr> <Th> Least populous state </Th> <Td> Sikkim (610,577) </Td> </Tr> <P> The 15th Indian Census was conducted in two phases, house listing and population enumeration . House listing phase began on 1 April 2010 and involved collection of information about all buildings . Information for National Population Register was also collected in the first phase, which will be used to issue a 12 - digit unique identification number to all registered Indian residents by Unique Identification Authority of India (UIDAI). The second population enumeration phase was conducted between 9 and 28 February 2011 . Census has been conducted in India since 1872 and 2011 marks the first time biometric information was collected . According to the provisional reports released on 31 March 2011, the Indian population increased to 121 crore with a decadal growth of 17.64% . Adult literacy rate increased to 74.04% with a decadal growth of 9.21% . The motto of census 2011 was' Our Census, Our future' . </P> <P> Spread across 28 states and 7 union territories, the census covered 640 districts, 5,924 sub-districts, 7,935 towns and more than 6 lakh villages . A total of 27 lakh officials visited households in 7,935 towns and 6 lakh villages, classifying the population according to gender, religion, education and occupation . The cost of the exercise was approximately ₹ 2,200 crore (US $340 million)--this comes to less than $0.50 per person, well below the estimated world average of $4.60 per person . Conducted every 10 years, this census faced big challenges considering India's vast area and diversity of cultures and opposition from the manpower involved . </P>

When was the last census done in india