Level of education was reclassified as compulsory school, middle level education (including vocational education below university level) and university degree.
Socioeconomic position was assessed through maternal questionnaire in pregnancy and included highest maternal educational attainment, we used four categories: university degree; A level or advanced level (which are after compulsory school qualifications); O level, or ordinary level (which are academic qualifications taken at the end of compulsory schooling.
Educational level was categorised into 'basic' (elementary and comprehensive compulsory school), 'mid-level' (residential college for adult education or high school) and 'high' (college or higher) educational levels.
Part two included questions on background variables; age, sex and education categorised as university, high school, compulsory school or "other" level that included adult education programs and vocational training.
Educational level was classified according to the International Standard Classification of Education ISCEDD): compulsory schooling (ISCED levels 0 2), secondary (3), higher secondary (4) and tertiary education (5 6) [ 24].
At the non-compulsory senior high school level, additional registration place-based constraints also apply because the education policy requires students to take the university/college entrance examination in their hukou registration area3.
Five items were dichotomized in order to discern negative implications of pain from insignificant implications of pain: 1) The item educational level was dichotomized to low when reported as 9-year compulsory school or upper secondary school levels and to high when reported as university education.
These curricula were designed for the three first school levels – preschool, compulsory school (6 16 years old), and upper secondary school (16 20 years old) – with the common aim of enhancing core competencies built on the six fundamental ideals for students (Ministry of Education, Science and Culture, 2011).
The parents of these young women represented a range of socio-economic status: 9.6 % of the mothers had completed compulsory school as their highest level, but only 1.4%% of the fathers.
The following predictor variables were included: socio-demographics – including sex, age (in years), household size (single, dual, 3+), highest level of education (compulsory school, apprenticeship or intermediate vocational degree, high school diploma, university) – subjective health status (1 = very good, 5 = very poor), and religious confession (Catholic, Protestant, Muslim, other, none).
