The article examines the quality and appropriateness of the data available to measure firms' affiliation to employer associations (EAs). We find large discrepancies in affiliation rates obtained from the five different data sources available for France, leading us in particular to discard tax data. Focusing on survey data, we show that asking managers about affiliation to EAs in general or affiliation to a list of specific EAs can lead to large differences in affiliation rates, highlighting the importance of the framing of survey questions. We then provide methods to estimate an aggregate firm‐level affiliation rate from surveys covering workplaces with 11 or more employees. Exploiting (i) conflicting survey responses regarding EA affiliation between distinct establishments in the same firm and (ii) survey responses for firms that report paying contributions to EAs in their financial statements, we finally estimate the shares of employers that wrongly declare being or not being affiliated to EAs, and provide a rate of affiliation corrected for such errors. The implications for econometric analysis of the high observed error rates are discussed.