This is actually a pretty interesting measure of immigrant selection of turks. "Anatolia's Revenge"http://www.unz.com/akarlin/anatolias-revenge/ …
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data may make it possible to take into account the Turk selectivity. But might also just reflect differential integration success.
Since you read Dutch, could you see if you could find me some better data than these? https://openpsych.net/paper/16 http://emilkirkegaard.dk/en/?p=6351