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'
database name:db_id authors
Author table Author Cols: Id dtype integer, Name dtype text, Affiliation dtype text, 
Conference table Conference Cols: Id dtype integer, ShortName dtype text, FullName dtype text, HomePage dtype text, 
Journal table Journal Cols: Id dtype integer, ShortName dtype text, FullName dtype text, HomePage dtype text, 
Paper table Paper Cols: Id dtype integer, Title dtype text, Year dtype integer, ConferenceId dtype integer, JournalId dtype integer, Keyword dtype text, 
PaperAuthor table PaperAuthor Cols: PaperId dtype integer, AuthorId dtype integer, Name dtype text, Affiliation dtype text, 
