Here's an interesting question:
What are the odds of walking if you don't swing at any pitches?
To answer that question I used the Lahman database and play-by-play files from 2003 to look at all plate appearances that were not intentional walks. Here's the answer:
Non IBB PA
BB No Swings Pct
K No Swings Pct
So about one out of 26 plate appearances a batter walks while not offering at a pitch. For this study "offerring" included swinging, bunting, foul tips, missed bunt attempts, swinging at intentional balls, and foul balls. I also found that striking out without offerring is actually more rare and happens only once in 150 plate appearances.