PREFIX dance: <https://w3id.org/rete/dance#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT ?skill (COUNT(?e) AS ?errors) WHERE {
?perf dance:hasSegment ?e ; dance:hasLeader ?l .
?e a dance:ErrorSegment .
?l dance:hasSkillLevel/rdfs:label ?skill .
} GROUP BY ?skill ORDER BY DESC(?errors)