Referring to USA, the place to go and learn a lot (if not everything) about the matter is The Bureau of Labor Staitstics
Indicatively, for one of BLS's many outputs, they report that:
Payroll survey — methods and measurement issues
The Current Employment Statistics (CES) program, also known as the
payroll survey or the establishment survey, is a monthly survey of
approximately 143,000 businesses and government agencies representing
approximately 588,000 worksites throughout the United States. From the
sample, CES produces and publishes employment, hours, and earnings
estimates for the nation, states, and metropolitan areas at detailed
The CES employment series are estimates of nonfarm wage and salary
jobs, not an estimate of employed persons; an individual with two jobs is counted twice by the payroll survey. The CES employment series
excludes employees in agriculture, private households, and the
...The entire sample is redrawn annually, and a supplemental sample of
new business births is selected midway through the year. About
one-fourth of the sample is rotated out each year and replaced with
newly selected businesses.
All new sample is solicited by computer-assisted telephone interview
(CATI), and data are collected for the first 5 months via this mode.
After the initiation period, many sample units are transferred to one
of several less costly reporting methods that are self-initiated by
CES monthly employment estimates are made using a two-part estimator.
The sample reports are used to estimate month-to-month employment
change from continuing businesses and a birth/death model is used to
account for new firm births that otherwise would not be sampled in a
CES uses automated edit and screening techniques to identify potentially erroneous sample data; respondents are
re-contacted as needed to validate or correct their reported
information. After the microdata edit process is complete, monthly
estimates are calculated. Automated edits of the estimates are
supplemented by analysts who look for errors and outliers and provide
final validation of the series before publication.
The seasonal adjustment process removes from the series the effects of
normal variation from recurring events within a year, such as holidays
and weather changes, and helps reveal underlying economic trends. CES
uses a concurrent seasonal adjustment methodology, meaning that it
incorporates estimates up through and including the current month’s
data to achieve the best possible series.
CES first preliminary estimates of employment, hours, and earnings are published each month approximately 3 weeks
after the reference period. Estimates are then revised twice before
being held constant until the annual benchmarking process. Second
preliminary estimates for a given month are published the month
following the initial release, and final sample-based estimates are
published 2 months after the initial release.