Measure of individual educational attainment is both one of the variables with difficulties in assessment in social surveys, and one among most commonly used variables for academic analysis. Difficulties are related to the frequent changes in institutional setting, combined with the retrospective character of information. In survey situation respondents report about the past educational level of last finished school. The difficulty arise with the self-classification task of reporting the education, which fits some pre-established limited number of categories in survey questions about education. Slovene survey tradition is reach in variations of methods of measuring education. Since 2011 a national register based Census introduced a new standard for measuring education. Classification System of Education and Training (KLASIUS) was used to convert on a common base the data of valid classifications of education of different periods, and contained in different sources. The classification used is established as 'Official Statistical Standard' that is compulsory for administrative and statistical evidences. It contains crosswalks to ISCED 1997. We will try to test the equivalence of conceptual meaning of different variants we found in past social surveys, and in Census, by assessing a construct validity of original variables and their harmonised counterparts. The results could inform some future planes for ex-post harmonisation of the past and current existing measures in surveys to increase a comparability of analytical results.