For the year that concluded on December 31, 1998, how many male patients on average were tested in the lab each month?
the year that concluded on December 31, 1998 refers to Date BETWEEN '1998-01-01' AND '1998-12-31'; male refers to SEX = 'M'; calculation = DIVIDE(COUNT(ID), 12)
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'