Write SQL query to solve given problem: What is the percentage of preprints of John Van Reenen's papers?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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'