SELECT avg(Years_working) FROM journalist
What is the average number of years spent working as a journalist?
[Schema (values) (types)]: | news_report |  event : event_id (text) , date (number) , venue (text) , name (text) , event_attendance (text) | journalist : journalist_id (text) , name (number) , nationality (text) , age (text) , years_working (text) | news_report : journalist_id (text) , event_id (number) , work_type (text);