Game Development Reference
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every time he defeats an orc. These are both fixed ratio schedules because
the ratio between action and reward is always the same.
The ratio doesn't have to be one-to-one. For example, a player might
get 10 gold pieces for every 10 th orc he defeats. In this case most actions are
not rewarded at all. The 10 th orc gives the reward that was earned over the
previous nine orcs.
By themselves, fixed ratio schedules are poor motivators. They encour-
age long periods of low inactivity, followed by a surge of activity when the
player decides he wants the next reward. Those long periods of inactivity
make it easy for the player to get up and walk away.
vaRiaBle Ratio
A variable ratio reinforcement schedule is like a fixed ratio schedule, except
that the ratio changes every time the reward is given. Usually, a variable
ratio schedule gives the reward after some randomized number of actions.
For example, a player might have a 10% chance of receiving 10 gold pieces
every time he defeats an orc. Depending on the roll of the dice, he could get
the reward three times in a row, or plow through 50 orcs and get nothing.
Variable ratio schedules are the most powerful simple reinforce-
ment schedule. Even when they give the same average return as a fixed
ratio schedule, they motivate very differently. A player facing a variable ratio
schedule always has a chance at a large reward on the next action, so activ-
ity stays high and consistent. You're always hoping the next orc will drop
the big payoff.
Variable ratio schedules are everywhere in games. Any game with sig-
nificant randomized elements has them to some degree, and all gambling
games are based on them. Some role-playing games make them the main
focus of play, with each monster and quest offering a different menu of
randomized rewards at different probabilities.
And they appear in life, too. Perhaps you've dated someone like this:
On some days, they are warm and welcoming. On others, they are cold
and distant. They don't return every call, and randomly forget to show up
when agreed. In short, they are playing hard to get. People who do this get
results for the same reason that slot machines do. It's just another variable
ratio reinforcement schedule.
otHeR ReinfoRCement sCHeDules
A fixed interval reinforcement schedule makes a reward available a certain
amount of time after it is acquired. In a Skinner box, the rat might only
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