Fixed interval: The presentation of a reward after a fixed amount of time (e.g., $1 every hour) Fixed ratio: The presentation of a reward after a fixed amount of effort (e.g., $1 every 5 questions answered) Variable interval: The presentation of a reward after a variable amount of time (e.g., $1 every 30-60 minutes) Variable ratio: The presentation of a reward after a variable amount of effort (E.g., $1 every 5-30 questions answered)