FUNDAMENTALS OF GAME DESIGN, SECOND EDITION
Factors Outside the Designer’s Control
In managing the difficulty of a game, you command a number of factors, but a few remain outside your knowledge or control. You cannot know how much time the player has already spent playing other games similar to yours—or, more accurately, facing challenges similar to those that you offer. This factor is called previous experience. (The experience the player gains while playing your game is called in-game experience.)
You also cannot know how much native talent the player brings to the game: hand - eye coordination, reasoning faculties, and so on. As a result, we can't include either previous experience or native talent in our calculation of perceived difficulty. We look into these factors when we come to the question of difficulty modes in the later section "Establishing Difficulty Modes."
Finally, in multiplayer games, the skill of the player's opponents plays the greatest role in determining how hard it is to beat them, and you do not control that.
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Consequently, you don't have to put as much effort into managing difficulty throughout the game, so long as the game is fair. You still have to set the difficulty of individual challenges posed by the environment in a multiplayer game, however. The height of the basketball hoop and the size of the rim determine how hard it is to shoot a ball through the hoop in absolute terms, but how hard it is to win the game depends on the quality of the opposing team.
Types of Difficulty
Players care most about perceived difficulty; what matters is how hard the player finds surmounting a given challenge. To design a challenge at your target level of perceived difficulty, you must take into account four factors: intrinsic skill required and stress, both introduced in Chapter 9, as well as power provided and in-game experience, defined shortly. We'll also examine absolute difficulty and relative difficulty, concepts that are helpful when you are trying to gauge in advance how difficult players will find the challenges you design for them.
To judge the absolute difficulty of a challenge, compare the amounts of intrinsic skill required to meet the challenges and the stress that the challenge imposes on a trivial challenge of the same type. For instance, in an action game, a trivial enemy would stand still, could not harm the avatar, and could be killed with one punch. If you design another enemy that takes more effort to kill (because it has more health points), that moves around (requiring more intrinsic skill to hit), and that hits the avatar back (thereby placing the player under time pressure—stress—to kill the enemy before the enemy kills the avatar), you can be confident you have designed
an enemy more difficult to defeat, in absolute terms, than the trivial enemy that established the baseline. In effect, the absolute difficulty of a challenge equals the intrinsic skill required and the stress of the challenge compared to the trivial case.
You will find the concept of absolute difficulty useful when you need to compare the difficulty levels of different challenges. In general, if one enemy has twice as many health points as another, all other things being equal, it survives twice as long under assault, making it twice as hard to defeat.
RELATIVE DIFFICULTY AND POWER PROVIDED
You cannot determine how the player perceives the difficulty of a challenge through absolute difficulty alone. You must also take into account two more factors. The first is the amount of power that the game gives to the player to meet the challenge. Power provided measures, by means appropriate to the situation, the player's strength: the health and powers of his avatar, the size and makeup of his army, the performance characteristics of his racing car, or whatever factors apply. In the simple example described in the previous section, power provided would refer to the amount of damage the avatar can do when hitting the enemy and the avatar's resistance to damage: the number of health points that he has to lose before dying.
The relative difficulty is the difficulty of a challenge relative to the player's power to meet that challenge. For example, in an RPG, a player playing a level 1 knight will find it much harder, in absolute terms, to defeat a large enemy than a small one.
But a player playing a level 5 knight won't find it nearly so hard to defeat that same large enemy because the game provides a level 5 knight with so much more power than it provides a level 1 knight.
If the power the game provides to the player doesn't change throughout the game, then you may ignore this distinction between absolute and relative difficulty. But most games include an upgrade progression whereby the player gains power as the game progresses because the new powers keep the player interested in the game and give her the feeling of accomplishing more. As a result, when level designers build challenges into the game world, they must also take into account the power provided to the player to meet those challenges. The level designers have to know, for example, that by the time the player reaches the fourth level, he will have earned three major weapon upgrades and a faster vehicle, so they set the difficulty of the fourth level's challenges relative to that level of power provided. To simplify managing the difficulty, many games don't allow the player to carry powers over from level to level; instead, the level designers themselves set the amount of power provided separately for each level and take it into account accordingly as they devise challenges. In persistent worlds, in which each individual player has his own amount of power provided, earned through his earlier play, the game must either warn players in advance against trying a mission that is too hard or flatly exclude them from such missions.
PERCEIVED DIFFICULTY AND IN-GAME EXPERIENCE
As they progress through a game, players learn to use the game's user interface more efficiently, and they learn at an intuitive level how the core mechanics of the game work. In-game experience at meeting any particular type of challenge may be measured by the amount of time the player has already spent meeting similar challenges within your game. (Remember that you cannot know how much previous experience the player has playing other games.) The more in-game experience a player has, the easier he perceives a given type of challenge to be. Thus, when the level designers build a challenge into a level, they must take into account the player's amount of in-game experience with the same type of challenge. If the player already has a lot of in-game experience with challenges of that type, the level designers should consider raising the challenge's absolute difficulty to compensate.
The perceived difficulty of a challenge—the difficulty that the player actually senses, and the type we are most concerned with—consists of the relative difficulty minus the player's experience at meeting such challenges. Remembering that relative difficulty is absolute difficulty minus power provided, we can put all these factors together into a single equation such that
perceived difficulty = absolute difficulty - (power provided + in-game experience)
Note that there are no units of measurement for these variables, so if you want to compute actual values for them, you will have to find a way to measure their quantities based on the challenges that you plan to include in your game. The equation serves more as a useful principle for you to understand than as a value you can really compute.
