Average rate of S ranks?

I was wondering how often people tend to get S ranks (S-, S or S+) on average? I have friends who seem to get them very frequently. Also most matches I see at least 1 or 2 players will get one, which would suggest the average rate is once every say 5-10 matches. However I rarely ever get them - got one for the first time the other day in like half a year, but that was playing with my friend who was leveling a smurf account.. so many of the players were probably quite new (low levels) so doesn't really count! I'm not very good at this game, but I would have thought that... since MMR is based on player skill, and S rank is supposed to compare you to other players playing that champ in that role around your MMR, that the rate should therefore be fairly consistent across skill ranges/divisions? Unless it doesn't take MMR into account.. or just not very much.
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