query: SELECT ?item ?sample_author ?count WHERE {
    {
    SELECT ?item (SAMPLE(?author) AS ?sample_author) (COUNT(?author)AS ?count) WHERE {
      ?item wdt:P50 ?author .
    }
    GROUP BY ?item ?sample_author
    HAVING (?count > 15)
             LIMIT 2000
                   }
    FILTER NOT EXISTS {?item wdt:P921 []}
  }
  LIMIT 200