Database: thrombosis_prediction
Question: For the year that concluded on December 31, 1998, how many male patients on average were tested in the lab each month?
SQL: SELECT CAST(COUNT(T1.ID) AS REAL) / 12 FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID WHERE STRFTIME('%Y', T2.Date) = '1998' AND T1.SEX = 'M'