A pattern probability (302) is determined for each of a number of target patterns (301) achievable in a bingo-type game. Each pattern probability (302) comprises a probability of achieving the respective target pattern (301) in the bingo-type game. Different pattern sets are then associated with each different prize level in a desired prize distribution. The target patterns (301) and their respective pattern probabilities (302) are assigned or mapped to the different pattern sets so that the probability of achieving any target pattern included in a pattern set comprises a value approximating the desired probability of the prize level with which the pattern set is associated.