Theory: Add Instead of Subtracting

Sometimes game design is about filing down rough edges, implementing things in ways that remove small but irksome play issues. I’ve run into one opportunity to do that recently: it’s often better to add than to subtract.

This might seem pretty trivial; after all, addition and subtraction are basic skills everyone learns in elementary school. However, it turns out that subtracting can lead to weird rules issues. Rather than have to deal with them, it’s often better to see if the same effect can be achieved through addition.

By way of example, consider a game where players roll dice and try to get above a certain number. (In other words, the vast majority of games with dice in them!) As the designer, you’ve decided that in certain situations the player should be less likely to succeed. Should you subtract from the player’s roll, or add to the total needed?

From a mathematical perspective, the two might be exactly the same. Subtracting, however, can create problems in extreme circumstances. What if the total of (roll – penalty) is less than zero? Does that have meaning?

Don’t laugh—it’s possible that a negative result could. In an economic game, for example, negative cost might serve as a way to reflect economies of scale. In a wargame based on ancient Greece, where morale was the most important factor for a defending army, a negative attack value might represent an attack so weak that it actually reinforces the defenders’ confidence in themselves.

If a roll of less than zero doesn’t have meaning, how exactly will it be handled? The value could just stop at zero, with a rule that it’s impossible to go lower. In that case it becomes necessary to address order-of-operations issues; if there are both penalties and bonuses to a roll, canny players might apply them so that some of the penalties are “wasted” by the not-below-zero rule.

At least one game I’ve played tried to avoid that problem by tracking negative values, but treating them as zero; the negatives only came into effect when a bonus tried to bring the total back up. The resulting system was mathematically workable, but somewhat hard to explain to new players. “You’re actually at -2, but we play like it’s 0, unless you try to increase it, in which case it’s -2.” Wrapping one’s brain around that while also trying to keep track of the basic game rules was not trivial.

Compare all of that to what happens if we just add to the total needed. In the abstract, that raises absolutely no rules questions. Nor can I think, offhand, of any specific game where it would.

Sometimes a game has to have subtraction. Keep in mind, though, that subtraction has a certain measure of built-in complexity. Where possible, use the mirror-image addition instead; it’s probably equally intuitive, and it will usually avoid creating FAQ entries.

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