star wars - What was C-3PO basing his odds on?

C-3PO: "Sir, the possibility of successfully navigating an asteroid field is approximately three thousand seven hundred and twenty to one!"

Han: "Never tell me the odds."

C-3PO: "The odds of successfully surviving an attack on an Imperial Star Destroyer are approximately..."
Leia: "Shut up!"

What was the basis of his calculations for the odds he was stating?