Calculate the average score per university under Alumni criteria in 2008.
under Alumni criteria refers to criteria_name = 'Alumni'; in 2008 refers to year = 2008; average score refers to DIVIDE(SUM(score), COUNT(university_id))
SELECT AVG(T2.score) FROM ranking_criteria AS T1 INNER JOIN university_ranking_year AS T2 ON T1.id = T2.ranking_criteria_id WHERE T1.criteria_name = 'Alumni' AND T2.year = 2008