German left-wing radicals say Jews have ‘too much influence’

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21. many surveys do not have control groups

Control groups are used in experiments. As for anonymous, do you think surveys about racism are not anonymous? That would definitely bias the results. As for ad hoc standard for leftist and criteria for anti-semitism, the study is clear in how they defined leftist - I would assume a well-known political science researcher would use established constructs and it seems the survey questions relied on those to gain an indication of holding of leftist views, and similarly used /defined constructs for holding of anti-semitic views. There is thorough statistical analysis to look at differences between subpopulations in the survey. This is what happens in research.

This is really a top notch scholarly research center there. To go out and criticize these people for not knowing what they are doing is the equivalent of how some passionate people criticize climate science because it is all just based on models that cannot be proven. Sounds a lot the same.

Really, you could just say "I don't understand survey techniques, but I trust my gut and my got doesn't like the idea that German leftists would be anti-semitic so I need more convincing than just a few surveys and a few rowdy crowds." Fair enough.