The bigger question... How do you explain this?
Obviously when you look at 55 individual pairings, the numbers are too small to really show a major difference. But in this case, you're looking at typically around 200 home games and 200 road games for each team. That should smooth out the variances.
Northwestern and Illinois don't seem to have strong HFA. That makes sense. Every game for Northwestern seems to be a road game because of opposing fans who have settled in Chicago coming to games, and their own fans' apathy. I'd venture Illinois might be similar... Lots of apathy in their own fan base and it's a short drive from Chicago or Indianapolis.
But how do you explain Ohio State having such a weak HFA? Ohio State has rabid fans who fill the 'shoe every game. The only thing maybe I can think is that it's similar to our tiers in basketball, where teams in the top tiers simply have more opportunity for negative upsets? I.e. that it's really hard to go THAT much better than 0.825 at home over 5 decades, so their dominance at home is capped?
Is that why IU also has the second-highest HFA despite being a basketball school? It's hard to do worse than 0.210 away, so their suckitude is capped as well? Yet that doesn't seem to affect Northwestern, who has a similar home win percentage of 0.378 to IU's 0.370, but doesn't have the same ineptitude on the road as IU...
And then there's Minnesota. Is it possible their HFA was due to playing in an indoor NFL stadium for almost 30 of those 50 seasons? Any chance you can look at their HFA from 1970-1981 plus 2009-2019, and then compare it to their HFA from 1982-2008?
I would expect that teams with more fan engagement like OSU would have higher HFA advantage than teams like IU who suck and whose fans are rooting for Notre Dame football while they wait for basketball to start... The fans certainly believe they're the "12th man", but clearly nothing in the table above justifies that fan volume is the key driver of HFA...
My guesses for some of these:
Look at Ohio State first. The wiseguys in Vegas generally say that HFA is around 3-5 points. Those points always matter to gamblers because whether Ohio State beats their hapless opponent by 27 (home) or 21 (away) is a big deal if you are betting on the game and Ohio State is favored by 24. However, for the rest of us I would venture to guess that nobody really cares whether Ohio State wins by 21 or 27. Either way it is a pretty substantial win bordering on a blowout.
We are only looking here at W's vs L's not point spreads so we couldn't care less whether Ohio State wins by 27, 21 or by an XP in OT. It doesn't matter because any of those three simply entered the above chart as one W.
Thus, I think that tOSU's HFA is rather mild simply because they were so good for most of those 50 years that there were not all that many games in which the difference was likely to fall within the roughly 3-5 point range where HFA could be decisive.
Similarly, the two schools with even less HFA than tOSU (IL and NU) were so bad for most of those 50 years that there were not all that many games in which the difference was likely to fall within the roughly 3-5 point range where HFA could be decisive.
One weakness of this theory is that Indiana should fall into the same category as IL and NU. They were generally awful over the 50 years in question (worst % both at home AND on the road) yet for some reason there was a substantial difference between their home winning % of .370 and their road winning percentage of just .210.
Here is another curiosity that I really can't explain, look at Ohio State and Michigan:
At home they were almost exactly equal:
- 162-34-1 for the Buckeyes
- 158-37-2 for the Wolverines
That is only a difference of 3.5 games over 50 years which is pretty clearly statistically insignificant.
On the road the Buckeyes were relatively much better:
- 143-45-4 for the Buckeyes
- 134-57-3 for the Wolverines
That is a difference 10.5 games over 50 years or a little better than one every five years.
It gets more inexplicable the more you think about it. I've travelled to every B1G stadium and while Michigan's is huge I've often referred to it as "the quietest 100k people you will ever meet". That isn't necessarily a knock on Michigan's fans. It has more to do with architecture. Unlike similarly sized two-deck stadiums such as the Horseshoe and Penn State's stadium in which the second decks hold in noise, Michigan's stadium is a giant bowl which just doesn't focus crowd noise on the field at all. It really is remarkably quiet compared to tOSU, PSU, UW, etc.
Based on that I'd have to assume that Ohio State gets a bigger boost from their crowd than Michigan does which would mean that since tOSU's and M's home records from 1970-2019 were effectively equal I'd have to concede that Michigan must have been putting a slightly better overall team on the field at home for that timeframe which was then made up for by Ohio State's somewhat louder home environment to yield an effectively equal home record. All of that would lead one to predict that Michigan should have been substantially BETTER on the road but they weren't. Why? I can't explain that. Maybe it is just because the home records for both teams (.807 for M, .825 for tOSU) are simply bumping up against the "dominance cap" that you referred to so it just doesn't matter after that.
