Bungie gives an example on their website that shows how it work, nonetheless, it is very complicated.
Going from a little experimentation and observations from my own and others, I believe to be on the closest trail to the answer about the ranking system. First of all bungie uses a separate system from live's trueskill matchmaking. So far, from reading bungie's halo 2 ranking sytem, i understand that in any game type the system does not rank you up according to kills or ratio and most likely on the halo 3 system that the same goes for mvp, medals, etc,. I do not know if this is true yet my experience makes me to believe that the percentage of wins to number of games played in a playlist affect your rank. i have had three accounts so far on h3 and i have noticed that after the first game i lost on my 2nd account whilst playing team slayer that i still ranked up by winning one game in a row. i also noticed on that account that after quitting near the end of game where i was winning and the team i was playing with won, that i went up a level. this makes me to believe that i didn't play enough games in this playlist for the system to bring me down a level cosidering i hadn't lost a game which validates my h3 ranking system theory about the win/total game percentage in a playlist. anyway on my first account i went on a losing streak playing w/noobs when i was at the skill lvl of 19 which downranked me to 11. from then on it took me 2 wins to rank back up and later when i was a 25 i went down 3 ranks consecutively which took me 3 wins in a row to go up 1 skill lvl and then 5 after that. so far i believe the system restricts that the slowest winning streak to rank up is 5. I have met no one that needs more than 5 consecutive wins to go up other than some reports about it taking 10 wins in a row to rank past 45. it does not necesarily need to be 5 consecutively but consider it like this: you have just reached 25, you either need 5 in a row or win 5 more wins than losses from 25 - 26. Example i win one game now i need 4 more to rank up. next game: i lose now i need 5 more. I win the next 4 then lose 1. now i need 2 wins in a row.. again i win 3 more rank up, then lose 1 and rank down. now i need another win to rank back up. If you prove this answer wrong plz tell me because i am still trying to figure it out but this has been the one answer that is worked countless time for others. it may just be 4 wins consecutively for you depending on how many games you have played on that playlist.
Currently, it takes me 5 in a row in team and im a 40 and trying to get to 50 on my first account. On doubles it takes me 4 wins. i have had a wierd phenomenon where in team doubles i went down 3 ranks and it took me 3 wins in a row to get to my previous rank and it went back to 3 wins.
Sorry for this being long. You question is very hard to answer.