The following is based on five years of data, I believe, in the linked article. I saw another article like this the other day that was for 12 years of college football data, and the success rate for end-of-game kicks was around 20%. These are the numbers from the linked article:
"For standard onside kicks, the success rate is around 18 percent; for surprise onside kicks, it is around 42 percent."
A "surprise" onside kick would be one that takes place
other than in the later part of the 4th quarter when a team is behind.
For example, didn't one team do a surprise onside kick at the start of the second half in a recent Super Bowl? That would be a "surprise" onside kick - one other than at the end of the game.
FOOTBALL OUTSIDERS: Innovative Statistics, Intelligent Analysis | Varsity Numbers: Fourth-and-15