Database: thrombosis_prediction
Question: From laboratory examinations in 1991, what is the average hematoclit level that is lower than the normal range.
SQL: SELECT AVG(T2.HCT) FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID WHERE T2.HCT < 29 AND STRFTIME('%Y', T2.Date) = '1991'