LOS ANGELES, Calif.–A three-day count of the homeless population in much of Los Angeles County will begin tonight in the San Gabriel Valley and East Los Angeles.

The effort, billed as the largest in the nation to tabulate the numbers of homeless, is conducted every two years, coordinated by the Los Angeles Homeless Services Authority.

Volunteers will count the homeless on streets and at hospitals, jails and foster care and mental health facilities, along with places not intended for people to live, such as cars, parks, sidewalks and abandoned buildings, according to LAHSA.

The count will cover about 4,000 square miles. It will exclude Glendale, Pasadena and Long Beach, where other agencies will count the homeless, Arnold said.

The count will be conducted in West Los Angeles and the South Bay Wednesday, the Antelope Valley Thursday morning and the San Fernando Valley, Santa Clarita Valley, metropolitan Los Angeles and South Los Angeles Thursday night.

“The 2013 Greater Los Angeles Homeless Count is unquestionably the most important exercise we do to learn who are the men, women and children amongst us experiencing homelessness and that’s why I commend the many volunteers, communities and organizations from all over Los Angeles for stepping forward to join LAHSA in this effort,” Mayor Antonio Villaraigosa said.

The count conducted in 2011 found a homeless population of 45,422 people, a drop of more than 2,000 from the 2009 count, when the tally was 47,572, according to LAHSA.

LAHSA says data collected over the next three days will be analyzed over several weeks to create a demographic survey that can be used to plan and deploy resources serving the homeless.

“We all have a stake in in preventing and ending homelessness,” LAHSA Executive Director Michael Arnold said. “The thousands of volunteers from communities all over Los Angeles participating in the count … demonstrate that people care enough about this issue to make a tangible commitment to do something about it.”