We classified education into two levels.
When classifying the years of education, we classified education into three levels.
We classified education as low (less than high school), middle (high school or vocational school), or high (college or above).
We classified education by the highest education received: primary school or the first 3 years of secondary school, complete secondary school, and higher education.
We classified education level as: less than elementary (≤6 years in full-time education), middle (7-9 years), high 10-12 yearss), and college and above (>12 years).
We classified education according to the International Standard Classification of Education as total years of formal education (United Nations Educational, Scientific, and Cultural Organization 1997), using three categories [≤ 10, 11 13, and ≥ 14 (reference) years of education] for the analysis.
We classified education according to the International Standard Classification of Education as total years of formal education, combining primary schooling and vocational or professional education in one variable (United Nations Educational, Scientific, and Cultural Organization 1997).
For comparison reasons we classified the education level as high education (university and higher vocational school), medium education (middle vocational school and higher or middle general secondary school), and low education (lower vocational school, lower general secondary school, primary school, or no education) [ 54].
Level of education was classified into: primary education (primary school or lower vocational education); intermediate education (lower secondary education or intermediate vocational education); and higher education (higher secondary education, vocational education, or university).
