The number of trade union members is based on the Structural Survey conducted by the trade unions. The survey was conducted every two years up to and including 2003 and the reference date was 31 March. Since 2005, the Structural Survey is held on an annual basis. Most trade unions have joined an umbrella organisation, e.g. the Federation of Dutch Trade Unions (FNV), the National Federation of Christian Trade Unions (CNV) or the Trade Union Confederation for Middle and Senior Management (MHP). Members of trade unions which have not joined one of these umbrella organisations are included in the category ‘Other trade unions’.
The trade union membership rate cannot be deduced from data provided by the trade unions. Statistics Netherlands does not ask survey participants whether they are employed or not, because trade unions have not incorporated this information in their membership records. The number of members given by the trade unions may refer to persons in paid employment, but also to persons not or no longer employed, unemployed, retired, etc. Trade union members living outside the Netherlands (e.g. embassy staff and military personnel) are included in the trade union data.