Calculate the average of authors for each paper from the year of 1990 to 2000.
Average refer to DIVIDE(COUNT(AuthorID where Year = 1990 BETWEEN 2000), COUNT(Title where Year = 1990 BETWEEN 2000))
SELECT CAST(COUNT(DISTINCT T2.AuthorId) AS REAL) / COUNT(DISTINCT T1.Title) FROM Paper AS T1 INNER JOIN PaperAuthor AS T2 ON T1.Id = T2.PaperId WHERE T1.Year BETWEEN 1990 AND 2000