Common Mechanisms - Game Mechanics: Advanced Game Design - page 116

Positive and Negative Feedback basketball
In Chapter 4, we briefly introduced Marc LeBlanc's concept of positive and nega-
tive feedback basketball to explain the effects of positive and negative feedback on
games. In this section, we'll discuss the idea in more detail and show how to model
it in Machinations.
Positive feedback basketball is played like normal basketball but with the follow-
ing extra rule: “For every N points of difference in the score, the team that is ahead
may have an extra player in play.” Figure 6.21 shows a model of positive feedback
basketball. It uses a very simple construction to model basketball itself: Every player
on a team has a particular chance to score every time step. Teams initially consist of
five players, so their chance to score is represented as a source with an initial pro-
duction rate of five (for simplicity, we assume that a basket is worth one point, not
two, and we ignore three-point shots and free throws). Next comes a gate with a
percentage; this indicates the percentage of player attempts that actually succeed in
scoring. As you can see, the blue team is much better than the red team is; the blue
team's chance of scoring is 40% while the red team's chance is only 20%. A pool
called Difference keeps track of the difference in the points scored, because every time
blue scores, a point is added to the Difference pool, and every time red scores, a point
is subtracted. Each team can field one more player for every five points that it leads
by—if it's ahead by five, it gets one more player; if it's ahead by 10, it gets two; and
so on. The development of the scores and the difference in the scores are plotted
over time in the chart. As you can see, the score of the better team quickly spirals
upward as the difference increases, allowing them to field more and more players.
FIGURe 6.21
Positive feedback
basketball. Team sizes
are prevented from
dropping below 5 by
setting their minimal
value to 5.
In negative feedback basketball, the extra rule is reversed: The losing, rather than
the leading, team can field an extra player for every N points of difference. Again,
this can be easily modeled as a Machinations diagram, simply by reversing the signs
of a few state connections. The chart that is produced by running the diagram is