Trade union density corresponds to the ratio of wage and salary earners that are trade union members, divided by the total number of wage and salary earners (OECD Labour Force Statistics). Density is calculated using survey data, wherever possible, and administrative data adjusted for non-active and self-employed members otherwise. Data are expressed in percentages and presented from 1980.

The database on Institutional Characteristics of Trade Unions, Wage Setting, State Intervention and Social Pacts (ICTWSS) has been developed by Prof. Jelle Visser at the University of Amsterdam. In its initial form, the ICTWSS database combined data from various sources and projects with a main focus on trade union in EU and OECD countries, collective bargaining and employment relations in Europe, and social pacts.