1957 - United Nations Statistics Division

SOURCES OF VARIATION
Completely adequate statistics of international population
movements would show the number of all persons
having crossed an international frontier during a given
period of time, the direction of their movements, and
their characteristics. The data for different periods, or
for different groups such as emigrants or immigrants, visitors
and residents returning, would be consistent for any
particular country, and comparable for different countries.
This implies that uniform or equivalent statistical
nomenclatures and methods of collection would be used,
such as are described in the Recommendations. National
series meeting those standards would readily fit in a set
of international tables, giving complete and unduplicated
representation of the movements of international travellers
and migrants.
Existing data do not generally meet the standards of
quality set forth in the Recommendations. They constitute
only fragments of the complete representation of international
migration and travel referred to above. Time
will be needed to achieve a substantial improvement in
migration statistics, since considerable administrative
difficulties will have to be overcome in many countries,
but steady progress is possible. In effect, noticeable improvements
have taken place during recent years in the
geographic and time coverage, scope, consistency and
comparability of statistics of international population
movements made available by the various countries for
publication in the Demographic Yearbook. A brief
analysis of the limitations to which existing data are
subject will now be given. 59
Sources of data
The materials presented in Tables 27-31 have been
obtained generally by means of a questionnaire sent to
governments by the International Labour Office. Supplementary
information was extracted by the Secretariat
from official publications and documents.
In the various countries, the data are drawn from a
variety of sources. These are shown in Tables 28-31 by
Roman numerals, in terms of a code which is also
used in the International Labour Office Year Book of
Labour Statistics. The code does not indicate degrees of
reliability. The classification of sources and their code
numbers are as follows:
I. Port statistics, provided by the shipping authorities
and based on ship or aircraft manifests or other administrative
documents made out by the navigation
companies or, in some cases, on individual forms
completed by each traveller or head of travelling
family group.
II. Statistics of frontier control, provided by the police
or immigration authorities supervising the crossings
of international frontiers. In some cases, the data are
based on individual forms.
III. Statistics based on coupons detached at points of de-
69 A detailed comparative analysis of the migration statistics of 69
countries or territories and an examination of the difficulties relating
to them are available in Problems of Migration Statistics. United
Nations, Department of Social Affairs, document STjSOAjSER.Aj5,
Lake Success, November 1949 (Sales No. 1950XUI.l). A discussion
of the problems of organization and operations in migration statistics,
induding possible applications of sampling methods, is presented in
International Migration Statistics, op. cit.
43
parture and arrival from identity documents issued
to migrants by their own country.
IV. Passport statistics; being the number of passports
issued to certain categories of prospective travellers,
these data are limited to national emigrants.
V. Statistics of local population registers, provided by
the authorities in charge of the registers, are based on
entries recording declarations of change of residence,
which are compulsory under the laws of a few countries.
VI. Other statistics, including those obtained in connection
with the enforcement of regulations concerning
immigration, work-permits for aliens, transport contracts
for emigrants, etc.
The diversity in the sources of statistics on international
popUlation movements corresponds, in a certain measure,
to the variation in definitions analysed above. This
diversity complicates the interpretation of different series
available for the same country and the making of international
comparisons. Uniformity in the method of collection
would therefore represent an improvement. In
paragraph 13 of the Recommendations, definite preference
is given to collection at the occasion of frontier
control for all countries except for the small number of
those where satisfactory data on international population
movements can be obtained from existing population
registers. The application of the recommended method
of collection would be facilitated by neighboring or
related countries making arrangements for the joint
compilation of the data. Paragraph 14 of the Recommendations
directs the attention of governments to this point.
An analysis of sources shows that outside Europe
migration data are obtained in seventy per cent of the
countries from reliable sources, frontier control and port
statistics, and popUlation registers. In Europe, a great
diversity of methods prevails, reflecting the particular
difficulty of collecting migration data in that continent
as a result of the multiplicity of frontiers crossed by large
numbers of travellers each year.
In general, no outright conclusion regarding the
quality of migration data can be drawn from the mere
indication of the source from which they were drawn. It
is necessary to analyze in some detail the collection procedures
followed by the country to determine what
limitations may result from the procedures. Furthermore,
in order to produce internationally comparable statistics,
collection methods need not be identical but should be
equivalent. In an island country, port statistics may give
results comparable to frontier statistics in a country
having only land borders. The latter type of data may,
under certain conditions, be comparable with statistics
from popUlation registers. Only a detailed study of each
case may reveal the reliability of the figures, and show
how comparable they may be for different countries and
periods of time.
Definitions and nomenclatures
Problems of definition and nomenclature are different
in statistics of international migration from those in
other demographic statistics. Population census and vitalregistration
data, at least elementary ones, relate to well
defined events such as the presence of a person in a
particular area, a birth or death. The crossing of an inter-