He was recruited to work on improving the census by one of his Columbia professors who was a special agent at the Census Office. In the 1880s he worked on the 1880 census, operating the Seaton device, which made tabulating census data easier by isolating and organizing the lines on large tallying sheets. It took eight years to count the 50 million Americans for the 1880 census.
After gaining an understanding of the census process and the idea to use punch representation from John S Billings, Hollerith used a job at the patent office to learn about machinery components, then began developing a mechanical solution to the census problem.
The Census Bureau held a competition in 1888 to find a more efficient system. The contest required participants to process 1880 census data from four areas of St Louis, MO, and the fastest results won a contract for the next census. The other two contestants captured the data in 144.5 and 100.5 hours, but Hollerith finished in just 72.5 hours. When they were challenged to break down the data and prepare it for tabulation, the other contestants took 44.5 and 55.5 hours, while Hollerith completed the task in 5.5 hours, winning him the contract.
He developed a mechanical tabulator based on punched cards to rapidly tabulate statistics from millions of pieces of data, which he received a patent for in 1889.
According to EDN
Van Nguyen (collect ) - WORLD RECORDS UNION - WORLDKINGS ( source of photo : internet )