What is the percentage of preprints of John Van Reenen's papers?
year = 0 means this paper is preprint; John Van Reenen is the author's name; papers refers to paper.Id; calculation = DIVIDE(SUM(paper.Id where Name = 'John Van Reenen' AND ConferenceID = 0 AND  JournalId = 0), SUM(paper.Id where Name = 'John Van Reenen'))
SELECT CAST(SUM(CASE WHEN T1.ConferenceId = 0 AND T1.JournalId = 0 THEN 1 ELSE 0 END) AS REAL) / COUNT(T1.Id) FROM Paper AS T1 INNER JOIN PaperAuthor AS T2 ON T1.Id = T2.PaperId WHERE T2.Name = 'John Van Reenen'