Thomas Jefferson directed this country's first census in 1790 when slavery was legal and women were not considered citizens.

In the 200 years that followed, the counting process has overcome obstacles and applied technological advancements to result in today's very extensive operation.

A census of population may be defined as the process of compiling demographic, economic and social data pertaining to all persons in a country or delimited territory, according to the 1958 United Nations Statistical Papers.

It also includes a count of "business establishments, housing units, farms or even governments themselves . . .," says Census '80, a textbook issued by the U.S. Bureau of the Census.

"The basic idea is to get a grasp on what is out there, what is happening to the population within different age groups," said Jeff Wingert, junior planner for the Centre County Planning Commission.

The census gives the government information it uses when creating and maintaining public policies, Wingert said. Government funding for certain areas is based on the population.

For example, the census shows the government the fate of Social Security or the health of the economy, Wingert said.

Governments have been recording the population for purposes of taxation since the times of Babylonia and ancient China, Egypt, Palestine and Rome. But ancient censuses only counted landowning citizens, according to Census '80.

"The first known counting of every man, woman and child occurred in central Europe in 1449, when Nuremburg, West Germany was enumerated because its leaders feared depletion of a limited food supply under a state of siege," Demographer Ralph Thomlinson said in Population Dynamics.

Demographers generally agree that the first continuing complete count taken at regular intervals was instituted in Sweden in 1749, the bureau said.

The U.S. census was started in the late 1700s because the Constitution ordered the population to have equal and adequate political representation, said Gordon DeJong, a University sociology professor.

The U.S. Constitution mandates that the population be counted every 10 years.

The 1790 census called for the name of the head of the family and the number of persons in each household within the following categories: free white males 16 years old and older; free white males under 16; free white females; all other free persons and slaves, the census bureau said.

The 1790 enumeration reported 3,929,326 people in the country.

The early enumeration ran into many problems, according to Census '80. First, the federal government did not distribute questionnaire forms, so enumerators in each state and territory had to provide their own and paper was very expensive.

Second, people were spread thinly across the country and were often difficult to reach.

Further, many people did not understand why the government was taking a census and were often suspicious, uncooperative and sometimes openly hostile toward enumerators.

The 1960 census was the first to make extensive use of the mail system to collect population and housing information.

By 1980, the census expanded to include population, manufacturers, mineral industries, agriculture, governments, business, housing, foreign trade, transportation and construction industries.

This year's census marks the first time that an extensive count of the homeless and illegal aliens will be taken, said Rose Ann Neary, district manager of the State College Census Bureau.

"We're trying to count everyone," she said.

The homeless count will be conducted in two operations, Neary said. "S Night," taken March 20, counted people living on the streets and living in homeless shelters. "T Night," taken Saturday, counted people living in temporary housing, such as welfare hotels.