I sort of agree with Bill. Saying a move "gains" X points suggests a few things to me, via the normal English language meaning of the word:
(#1) The difference in how much a position is "worth" before and after the move is X points.
(#2) If I were to pass instead, I should get a final result X points worse on average, because obviously a pass "gains" 0 points instead of X points.
It's not an absolute requirement of course, but it would still be nice to have the word "gain" mean something consistent with the above two intuitions. And in double-gote positions in real games, both of these intuitive meanings are satisfied by the
half the swing value:
- Click Here To Show Diagram Code
[go]$$B
$$ . . . . . . . . .
$$ . . . . . . . . .
$$ . . . . . . . . .
$$ . O O O O O O O .
$$ . X W X . X B X .
$$ . X a X b X c X .
$$ . X X X X X X X .
$$ ---------------[/go]
In situation "a" black has 0 points, in "c" black has 1 point. In situation "b", in real game situations like this, on average either player is equally likely to get it first. So on average black has 1/2 point.
If black does spend a move in situation "b", black will turn it into position "c". Black's move
gains half of a point because before they had 1/2 point on average, afterward they have 1 point. The difference in how much these positions is worth is half a point. (matching intuitive meaning #1 above)
If black were to pass rather than playing, on average black would do 1/2 point worse. For example, if there were an odd number of positions like this left and nothing else, black end up a full 1 point worse. But if there were an even number of them, black would not end up worse at all for passing. In a real game, it's basically random whether there will be an even or odd number, so on average, black will be 1/2 point worse off by passing. (matching intuitive meaning #2 above)
Similarly, if you took random pro games with a position like "b" on the board (where neither black nor white had any liberty or eye issues, both white and black were already 100% alive), and let white get a free stone at "b" near the start of the endgame, not spending their turn, and then had both players finish the endgame normally and count, on average black would do 1/2 point worse, not 1 point worse. (another variation on intuitive meaning #2 above)
Explaining this requires precision on the part of the teacher, but I think if the teacher is precise, it's not particularly hard for the beginner to understand gains rather than swing values. When I teach beginners in-person, I do actually make sure to be precise when I explain move values, and the people I've taught seem to understand pretty well.
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So back to the position at hand:
- Click Here To Show Diagram Code
[go]$$
$$ | ? ? O ? ? ?
$$ | ? ? O ? ? ?
$$ | O O O , ? ?
$$ | a X X X X ?
$$ | O O X ? ? ?
$$ | . O X ? ? ?
$$ +------------[/go]
With this position, if I was going to very wordily explain it in real life to someone, I'd say something like:
"If black plays first, black has 7 points in the corner. If white plays first, white has 1 point, which is the same as (-1) for black. If neither player has played here yet and is busy with fighting elsewhere on the board, we expect on average 3 points for black here since knowing nothing else, either player might get to move first. So relative to that, the value/gain/worth of a move here is 4 points. If black plays first, black will do 4 points better than that (7 points) and if white plays first, white will do 4 points better than that (-1 points). Either way, the value of a move here is
4 points."
And also in practice, if I was continuing to try to explain, I'd also say something like:
"Many people you talk to might call this 8 points, because they're counting the
total swing between black playing first and white playing first as 8 points. Note that the 8 points is actually over
two moves, yours and your opponents. For example, if it was your turn, and you passed up playing here to play somewhere else instead, and then your opponent then played here instead of you, you'd get to play somewhere else
again, - you would be a total of 8 points worse here compared to playing here yourself, but you'd have gotten
two moves elsewhere instead, not just one, so the
per-move gain/loss is still 4.
Per-move values work better once you get into more complex positions that don't get finished in one move or two moves, or you start getting into cases where moves might be sente or forcing. But lots of people will talk in terms of total swing. If you personally also find it easier to think in terms of total swings for the simplest cases like this, that's also okay, just be aware of the difference and make sure you're consistent".