Question: Calculate the average number of criterias among "Times Higher Education World University Ranking","Shanghai Ranking" and "Center for World University Rankings".
SQL: SELECT (SUM(CASE WHEN T1.system_name = 'Center for World University Rankings' THEN 1 ELSE 0 END) + SUM(CASE WHEN T1.system_name = 'Shanghai Ranking' THEN 1 ELSE 0 END) + SUM(CASE WHEN T1.system_name = 'Times Higher Education World University Ranking' THEN 1 ELSE 0 END)) / 3 FROM ranking_system AS T1 INNER JOIN ranking_criteria AS T2 ON T1.id = T2.ranking_system_id