Conversation so far:
Was Letterman really popular?
In 1993 and 1994, the Late Show consistently gained higher ratings than The Tonight Show.
How many viewers did he have?
Letterman in the ratings by a 1.3 million viewer margin (5.2 million to 3.9 million).

Question: How was his rating?
Self-contained rewrite: How was David Letterman's ratings compared to the Tonight Show?