By Paul Theerman, Director
Every 10 years, the Federal census counts the country’s population. The count is mandated in the Constitution in order to distribute political power, as the census leads to deciding how many representatives a state will send to the House as well as to redrawing their district boundaries. Government resources flow according to population. And public health research uses census data, providing tools to better understand the conditions of people and their health.
Using census data for social purposes relies on a particular way of measuring things, though. To be useful, some “granularity” is needed: it’s not just at the state or county level that we need statistics, it’s rather at the block and neighborhood level. And there needs to be some sense of “commensurability”: a measure of a neighborhood in one part of a city, say, needs to be readily comparable to the measure in another part. And stability of the measuring unit is important; the geographical unit needs to stay the same over the years. For populations that are ever-changing, in place and age and origin, this is no mean feat. The fact that it works in the American context at all is largely due to one man, Walter Laidlaw, a statistician of the early 20th century who revolutionized the way that the Census Bureau carried out the New York census, a change that eventually was implemented for the whole country.
Walter Laidlaw (1861–1936), was a Canadian Presbyterian minister. As a child, he was adopted by his uncle, Robert Laidlaw, founder of a prominent lumber company in Esquesing Township, southwest of Toronto. After graduating from the University of Toronto in 1881, Princeton Theological Seminary in 1884, and going on for further study at the University of Berlin and again at Princeton Seminary, he was called to be pastor of the Jermain Memorial Church in Watervliet (now West Troy), New York, a post he held from 1886 to 1892.
After a year as president of the new University of Fairhaven (which later became Western Washington University, Bellingham) from 1892 to 1893, he settled in New York City, at St. Nicholas Collegiate Reformed Church, part of the Dutch Reformed tradition, from 1894–1895. But a new opportunity called: in 1895, he was appointed the first executive director of the newly formed New York Federation of Churches and Christian Workers. He held this position for almost 30 years, until 1922. Along the way, Laidlaw earned a Ph.D. from New York University in 1896; the field has not been determined, but statistics was his passion.
From his position as executive director of the Federation of Churches, and as editor of its journal, Federation, Laidlaw sought to put the work of religion on a secure scientific basis. Who were the people of New York? What social and economic needs did these people have? Where were the (Protestant, at least) houses of worship and settlement houses? Data were needed, and the census seemed a good place to start.
At this time, there were two: the Federal census, conducted in the years ending in “0,” the New York State census in the years ending in “5,” and they didn’t work the same way. As he details in a classic 1906 article in Federation, the counting principles differed between the Federal and state censuses, from one year’s census to the next, and even for different parts of the city within the same census. The Federal census of 1900 rolled up its data differently: for Brooklyn, Queens, and Staten Island, city council wards were used, and for Manhattan and the Bronx, New York State Assembly districts. And then the 1905 state census used New York State Assembly districts in all five boroughs. In Laidlaw’s words: “The ward is a fixed boundary, immobile as the orthography of a dead language,” he said; drawing out the metaphor, he continued, “the Assembly district is a changing boundary, a phonetic spelling arrangement which responds to the alien accents in the makeup of the city.” To get good data, the Federation found itself retabulating first the 1900 Federal census for 2 boroughs, and then, for the 1905 state census, for all 5 boroughs. At this point, Laidlaw called for a new system: “The scientific sociological study of Greater New York requires a ‘dead language’ boundary for tabulations. . . . Federation respectfully suggests a scheme which does away both with ward and Assembly district outlines, and which can be permanent.” 
The system he proposed was securely within the American tradition. It was, in fact, to use the system that was enshrined in law in the Land Ordinance of 1785: the “section” system set up to survey and sell the undeveloped lands west of the Appalachians. (An arial view of a Midwestern county, in Indiana, say, would reveal the regularity of the system!) Laidlaw’s first unit of analysis was the quarter-section: a quarter of a square mile, or 160 acres.
The whole city could be mapped into 1,308 quarter section plots, . . . The “quarter sections” could not, to be sure, be invariably 160 acres. Blocks should not be broken. But well defined area of about 160 acres could easily be devised. . . . [The] designation . . . could become uniform in the Federal Census tabulations, perpetually, and in the work of every department of the city, not excepting even the tax office.
The idea caught on. The U.S. Census Bureau adopted it the same year in preparation for the 1910 Federal census, but took the area down to 40-acre plots and called the basic units “census tracts.” By 1914, the city’s Department of Health had adopted census tracts as “‘sanitary areas’ to be followed in constructing new administrative districts in tuberculosis clinic work, baby health station districts, etc.” The sanitary areas were grouped together to form “Health Area Units.” 
Although eight cities ostensibly made use of census tracts in 1910 data gathering, New York remained the only one to analyze and publish the data to that granularity. The technique spread to 10 more cities for the 1930 census, and 42 more in 1940. The entire county was covered by census tracts by the year 2000. More importantly, the social service purposes that Laidlaw brought to his recommendations for the census came to fruition in governmental circles. Census tract data became a standard unit for analysis in public health, both for government and for academia.
As for Walter Laidlaw, he was deeply involved in every census from 1910 to 1930, often as a leader in the city census committees and as editor of the published compiled results. On May 20, returning from lunch at the Mayor’s Committee on City Planning, he died. His funeral, at Riverside Church, presided over by Harry Emerson Fosdick, attracted the notables of the City.
 “Laidlaw, Walter.” Who’s Who in New York (City and State): A Biographical Dictionary of Contemporaries. 6th biennial ed. Ed. William H. Mohr (New York: Who’s Who in New York City and State, Inc., 1914), 434–435. https://archive.org/details/whoswhoinnewyor1914hame/page/n79/mode/2up]
 Laidlaw, Walter. “Federation Districts and a Suggestion for a Convenient and Scientific City Map System.” Federation 4(4) 1906: 2–6.
 Godias J. Drolet and William H. Guilfoy, “Organization of Local Health Area Statistics in New York City,” American Journal of Public Health 20(4), April 1930: 380–386.
 New York Times, May 23, 1936; p. 15.