The framework necessary for designing and constructing ISCO-88 has been based on two main concepts: the concept of the kind of work per-formed or job, and the concept of skill.
Job - defined as a set of tasks and duties executed, or meant to be executed, by one person - is the statistical unit classified by ISCO-88. A set of jobs whose main tasks and duties are characterised by a high degree of similarity constitutes an occupation. Persons are classified by occupation through their relationship to a past, present or future job.
Skill - defined as the ability to carry out the tasks and duties of a given job - has, for the purposes of ISCO-88 the two following dimensions:
(a) Skill level - which is a function of the complexity and range of the tasks and duties involved; and
(b) Skill specialisation - defined by the field of knowledge required, the tools and machinery used, the materials worked on or with, as well as the kinds of goods and services produced.
On the basis of the skill concept thus defined, ISCO-88 occupational groups were delineated and further aggregated.
Bearing in mind the international character of the classification, only four broad skill levels were defined. They were given operational definitions in terms of the educational categories and levels which appear in the International Standard Classification of Education (ISCED), COM/ST/ISCED (Paris, Unesco, 1976).
The use of ISCED categories to define the four skill levels does not imply that the skills necessary to perform the tasks and duties of a given job can be acquired only through formal education. The skills may be, and often are, acquired through informal training and experience. In addition, it should be emphasised that the focus in ISCO-88 is on the skills required
to carry out the tasks and duties of an occupation - and not on whether a worker having a particular occupation is more or less skilled than another worker in the same occupation.
Therefore, as a rule, the following operational definitions of the four ISCO-88 skill levels apply where the necessary occupational skills are acquired through formal education or vocational training.
(a) The first ISCO skill /eve/was defined with reference to ISCED category 1, comprising primary education which generally begins at the age of 5, 6 or 7 and lasts about five years.
(b) The second ISCO skill level was defined with reference to ISCED categories 2 and 3, comprising first and second stages of secondary education. The first stage begins at the age of 11 or 12 and lasts about three years, while the second stage begins at the age of 14 or 15 and also lasts about three years. A period of on-the-job training and experience may be necessary, sometimes formalised in apprenticeships. This period may supplement the formal training or replace it partly or, in some cases, wholly.
(c) The third ISCO skill level was defined with reference to ISCED category 5, (category 4 in ISCED has been deliberately left without content) comprising education which begins at the age of 17 or 18, lasts about four years, and leads to an award not equivalent to a first university degree.
(d) The fourth ISCO skill level was defined with reference to ISCED categories 6 and 7, comprising education which also begins at the age of 17 or 18, lasts about three, four or more years, and leads to a university or postgraduate university degree, or the equivalent.
Unavoidably, some subjective judgement was involved in determining the skill levels of occupations, or occupational groups, in the structure of ISCO-88. Many national classifications and national circumstances have been examined to gather data for this purpose, and it is hoped that the decisions made reflect prevailing situations and main trends.