Vinnumarkaður - 01.06.1995, Side 27

Synopsis of methods and concepts
25
of unemployed in the year 1994 is estimated as 7,700, the
confidence limit for the number which comes nearest to this
estimation is ± 1,000. This means that in 19 surveys out of
20 the mean unemployment in 1994 would have been
estimated between 6,700 and 8,700.
Estimates with relative standard error exceeding 20% are
marked with an asterisk (*), cf„ e.g., Table 2.4.
Non-sampling errors. There are three categories of non-
sampling errors: Coverage errors, non-response errors and
measurement errors.
Coverage errors. The sampling frame comprises persons
with domicile in Iceland. A certain portion of these reside
abroad for more than 6 months, however, because of work or
study. Only a small number of these people actually register
their foreign residence at the National Register. This results
in an over-coverage of the survey population and a corre-
sponding bias in estimates. All estimates have been cor-
rected on basis of the sample to remove this bias (cf. Table
2.4).
Under-coverage errors have not been detected to any
degree.
Non-response errors. Males are more difficult to reach
than females, both because of absence from home or non-
contact. Young people have a higher non-contact rate than
older people and inhabitants of the capital region are more
difficult to contact than others. Refusals are more prevalent
among women, inhabitants of the capital region and older
persons. Response rates are, however, higher for women,
people outside the capital region and middle aged persons as
these are less likely to be away from home or not to be found
(Table 2.5).
To counter possible non-response bias the results have
been weighted by sex and age-group information from the
National Register. No use was made of residence data, as
these are less reliable than other data.
It is more difficult to adjust for non-response bias directly
related to the subject matter of a labour force survey. Certain
occupational groups are more difficult to contact than oth-
ers, e.g., fishermen. From November 1993 this has been
dealt with by putting the questionnaire to close family
members of these people. The number of proxy answers is,
however, only about half percent of all responses.
In most cases the item non-response has been corrected in
order to preserve consistency in totals as well as to counter
bias resulting from non-response. Two main methods have
been used. Firstly, a predicted answer has been deduced
from the answers of similar respondents. Missing data on
working hours were, e.g., replaced by regression coeffi-
cients from a regression analysis using sex, full-time/part-
time employment and occupational group as independent
variables. Secondly, the most probable response category
has been deduced from other responses by the respondent
either in the same survey or other surveys in which he or she
participated.
Measurement errors. Measurement errors are classified
into three categories in this context: Interviewer errors,
processing errors, and design errors.
Interviewer errors are those resulting from wrong record-
ing of answers, omission of questions, wrong routes in the
questionnaires or the rephrasing of questions by an inter-
viewer which results in a change of meaning of the ques-
tions. These errors were more pronounced before the com-
puterisation of the interviewing process.
These errors have mainly been dealt with by hiring only
experienced interviewers, by interviewer training and spe-
cial meetings with interviewers before each survey.
Processing errors. Before computerisation of the survey
in November 1992, errors sometimes happened when paper
questionnaires were entered into computer files. Coding of
certain open questions, especially regarding occupation,
economic activity and educational level are also prone to
errors.
These errors have mainly been dealt with by using expe-
rienced coders, by double coding of these variables and
systematic searches for inconsistencies.
Design errors. During the first three years, the question-
naire has undergone changes, although the main body of
questions has stayed the same. This is especially true with
regard to the phrasing of certain questions and their se-
quence. During the period many questions have also been
added in order to conform more fully to the requirements of
the EEA agreement. Bias related to these sources is not
known.
As the labour force survey is only conducted twice each
year, estimates of totals which are subject to seasonal
variations can differ from the true totals. Using registered
unemployment in April and November 1975-1994 as com-
pared with the annual averages in this period as a bench-
mark, the estimates for unemployment seem to be somewhat
conservative on the average and approximately non-biased
during the period from 1988-1994 (cf. Figure 2.1).
Finally, as each rotation group has been selected by a
simple random method with equal sampling fraction across
cohorts, the two youngest cohorts have been underrepresented
in the sample. This results in biased estimates of the total
labour force participation rate as these cohorts have lower
labour force participation than the average. This has been
dealt with by using a weighting scheme with age groups 16,
17, 18-19 and thereafter at five-year intervals.
3.1.4. Concepts
Main activity. In four tables (Tables 6.58-6.61) the data are
classified according to main activity status, which differs
from the ILO recommendations. Employed persons in these
tables are those whose usual hours of work are 12 hours or
more per week, whereas unemployed people are those who
are actively seeking employment, with the exception of
students who are only looking for a part-time job. Economi-
cally inactive persons are classified according to their
reported status.
Age. Age of respondents as of the 15th of each survey
month.
Economic activity. Economic activity is classified ac-
cording to NACE (Rev. 1) at the four-digit level. However,
only sections A to Q are reported. Section C, Mining, is
collapsed with section D, Industry, and secdons P and Q are
reported together with section O. Because of its importance,
the fishing industry is reported especially as a subcategory
of section D.
Underemployment. If a respondent worked less than 40