Answers to Why do many Mars missions launch now, if the Hohmann transfer orbit is the most propellant-saving one? and discussions below them made me wonder what makes Earth-Mars transfers more non-Hohmann; inclination or eccentricity?
So I've asked.
The answer might be different between Earth-to-Mars and Mars-to-Earth, then again, it might not!
Maybe it depends on the year? If so, I think which is more often the reason or "on average" might be good enough, but someone may be able to dig up a table somewhere, I can't be the first person to wonder this, and it can be addressed quantitatively with a fairly simple analysis provided a definition for "non-Hohmannness" can be established.