text: Moreover, there can be**major comparability difficulties**caused by the change of methodological 
  approaches, geographical coverage and other context factors during the last 10 to 30 years. Major 
  challenges for comparative assessments of EU-wide harmonised data collections from different years 
  were: 
  
  • The EU went through**several enlargement processes**, expanded from EU-12 to EU-15 in 
  1994, expanded from EU-15 to EU-25 in 2004, to EU27 in 2007 and to EU28 in 2013, and 
  from 2020 on — due to the departure of the United Kingdom — the EU consists of 27 Member 
  States. In statistical publications the identifier EU27_2020 is often used to distinguish this 
  period from the EU27 phase between 2008 and 2012, before Croatia joined and the EU27 
  became EU28. 
  
  •**Methodologies of data collection changed**, questions in surveys were abandoned or 
  
  changed, and sample sizes or structures changed, for example, the given period in survey 
  questions changed. One example is from the EWCS: the time categories for health-related 
  absence from work changed from ‘between 10 and 20 days’ to absence of ‘more than 15 
  days’. 
  Important**structural decisions were taken in the sector of economic statistics**, like the 
  change of the statistical composition and the coding of economic sectors, NACE Code 1, 
  Revision 1 (NACE 1.1) was applied until 2007, and from 2008 NACE Code 2 is applied. 
  
  • 
  
  • The survey providers use(d) for**occupation and educational attainment different**
  **categories**and aggregations levels, for example, ESEG, ISCED or ISCO. 
  
  • Some important categories and definitions are**not fully harmonised**in statistics, for example, 
  the definition of ‘manual worker’ or of ‘migration status’.468 