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Shifting US ideologies. Notice how Republican ideology is going outward from the centre since approximately 1975.
Ideology here is measured according to the DW-NOMINATE scale, a score that ranges from -1 (extremely liberal) to +1 (extremely conservative) based on legislative votes, and is designed to be comparable across time. (The source data is available for download at voteview.com.) The width of each band represents the spectrum of all but the 10% most extreme legislators within each party; the dark dot shows the median.
