In the extensive discussion of the ratings of cards, one of the most important topics that comes up is: What makes one card "better" than another? Is it the probability that it will serve any function during a game? Is it the probability that it will be make a big impact on the game? Is it the degree of the impact it makes?
I propose introducing the idea of a "key card": a Kingdom card that was important to its deck. For the purposes of analysis, I'll define below a mechanical way of determining key cards. One important note is that a deck can have multiple key cards. For example, in a Duke/HT game, both Duke and Horse Traders will be key cards. (Conversely, in a BOM game, a deck may have no key cards.)
My imagined uses of this "key card" idea are: first, looking at the overall statistics, we can rank cards as a fraction of (times it appears as a key card in the winning deck) / (times it appears in the Kingdom). Therefore, we can rank cards by how often they are an important part of the winning strategy. Second, for an individual player, we can rank cards in the same way, for both wins and losses. Therefore a player can see what cards they tend to succeed and fail with.
A Kingdom card is a key card if it meets ANY of the following criteria:
1. The player voluntarily gained at least 4 of the card.
2. The player voluntarily gained at least 2 of the card, and not more copies of any other Kingdom card.
3. The player did not voluntarily gain any card of equal or greater value first.
4. The player gained the card on Turn 1 or 2.
Note that none of the rules require that the card stay in the deck--if it is trashed, passed, or returned to the supply, the gain is what counts.
Rule 4 is to ensure that cards like Courtyard, Chapel, Masquerade, and Ambassador can count as key cards in, say, a Silver/Chapel opening where the Kingdom card is typically bought second.
For the purposes of Rule 3, Peddler is a $0 card. (If Peddler is indeed an important card in a deck, Rule 1 or 2 should cover it.)
For Black Market, I propose the guideline that every card purchased using a Black Market counts as a Black Market for Rules 1 and 2.
To test whether these rules are good, I examined a few of my recent games. I have called out the key cards in the winner's deck and, to differentiate this from the naive method of simply examining what was gained, cards that were gained but not key cards. I have chosen my most recent games as of today, leaving out only ones that were resigned.
http://councilroom.com/game?game_id=game-20120729-203828-ca73603f.htmlBaron and Fool's Gold. Not Herbalist.
http://councilroom.com/game?game_id=game-20120729-142344-41d900e7.htmlCartographer only. My opponent brain farted his Golden Deck; if he had not, he would have won and it would have been Lookout and Bishop.
http://councilroom.com/game?game_id=game-20120729-140110-cb33ac4b.htmlBridge, Festival, Embargo, Vineyard. Not Noble Brigand. Embargo looks a little off here, but they kept my opponent's Pirate Ships in check, provided cash, and provided a cheap pile to destroy. Vineyard was not a big deal, but if my opponent had gone for Provinces, it might have become one.
http://councilroom.com/game?game_id=game-20120729-134926-023a437f.htmlJack of all Trades, Laboratory, Bank; not Nobles
http://councilroom.com/game?game_id=game-20120729-134313-c9ca1c6c.htmlSmithy and Farmlands; not Grand Market or Explorer. I absolutely agree that Farmlands was critical here; note the 1-point victory margin.
http://councilroom.com/game?game_id=game-20120729-133711-5ffec986.htmlRemake, Mountebank, and Worker's Village; not Market, Scheme, Steward, Cellar, or Militia. The omission of Scheme is my biggest concern, but otherwise I agree.
http://councilroom.com/game?game_id=game-20120729-132325-d64af0f0.htmlJack of All Trades only, unsurprisingly; not Haggler, Forge, Tunnel.
http://councilroom.com/game?game_id=game-20120729-131709-748d6103.htmlGardens, Oasis, Minion; not Apprentice, Lighthouse. Gardens is a weird choice here because my opponent meant to buy Island instead, and I still lost horribly. Minion and Oasis, definitely.
http://councilroom.com/game?game_id=game-20120729-070702-62f6fb83.htmlFool's Gold, Festival. The 5/2 split was good to me.
http://councilroom.com/game?game_id=game-20120729-070206-f5cfe296.htmlTrading Post, Hamlet, Goons, Crossroads, Mining Village. Not Swindler, Trade Route, Royal Seal. The rules work well here, with the possible exception of Crossroads, which I attacked to end the game.