The Pew Research Center employs three expert survey measures to categorize European political parties as populist. These measures include anti-elite ratings from the 2024 Chapel Hill Expert Survey (CHES), the 2023 Populism and Political Parties Expert Survey (POPPA), and the 2023 PopuList.

A political party receives a populist classification when at least two of these three measures identify it as such. The CHES survey, conducted from October to December 2024, involved 609 political scientists specializing in European integration and political parties. These experts assessed 279 European political parties across all European Union member states, excluding Luxembourg, and also included parties in Iceland, Norway, Switzerland, Turkey, and the United Kingdom.

CHES utilizes an average of two variables to measure anti-elitism: 'PEOPLE_VS_ELITE' and 'ANTIELITE_SALIENCE'. The 'PEOPLE_VS_ELITE' variable uses a scale from 0 to 10, where 0 indicates support for decisions made by elected officeholders and 10 indicates support for 'the people' making decisions. The 'ANTIELITE_SALIENCE' variable measures the prominence of anti-establishment rhetoric on a 0-to-10 scale, with 0 meaning not salient and 10 meaning extremely salient. Parties with an average score of 7.0 or higher on these CHES anti-elitism measures are defined as populist.

The POPPA survey, conducted between January and May 2023, surveyed 324 experts on 312 political parties in 31 European countries. POPPA uses a 'POPULISM_MEAN' measure, which is an average of five indicators: 'MANICHEAN,' 'INDIVISIBLE,' 'GENERALWILL,' 'PEOPLECENTRISM,' and 'ANTIELITISM.' Parties scoring 7.0 or higher on the POPPA 'POPULISM_MEAN' measure are defined as populist.

The PopuList is a project that classifies European political parties into categories such as populist, far right, or far left. This project focuses on parties that have secured at least one seat or 2% of the vote in a national parliamentary election since 1989, and the appendix uses The PopuList 3.0. Traditional and populist parties are further categorized into three ideological groups: left, right, and center. This ideological classification uses the 'LRGEN' variable from the CHES dataset, where 0 represents the extreme left, 5 is the center, and 10 is the extreme right.

Parties scoring below 4.5 on the CHES ideology scale are defined as left parties, while those scoring above 5.5 are defined as right parties. Center parties register ratings between 4.5 and 5.5 on this scale. The POPPA dataset also assesses party ideology through the 'LROVERALL' variable, which ranges from 0 (left) to 10 (right). POPPA defines left parties as those scoring below 4.5 and right parties as those scoring above 5.5. Furthermore, POPPA categorizes parties with ratings between 4.5 and 5.5 as center parties. POPPA was used to confirm CHES ratings, showing full agreement on parties classified as populist using the described methodology.