The full article is here: www.theguardian.com/world/ng-interactive/2018/nov/20/revealed-one-in-four-europeans-vote-populist
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To conduct the research, the Guardian enlisted the help of more than 30 political scientists to assess which European parties could be considered populist, under academic definitions of the word, at various points over the past 20 years. Matthijs Rooduijn, a political sociologist at the University of Amsterdam, oversaw the project, and made the final decision on all party classifications.
Combining these classifications with the University of Bremen’s parlgov.org database of election results (which includes all parties winning parliamentary representation, or in some cases all parties winning more than 1% of the vote), we collated the aggregate performance of populists year-by-year over the period.
Our full methodology is here: www.theguardian.com/world/2018/nov/20/measuring-populism-how-guardian-charted-rise-methodology