Cell phone companies offer many plans with different amounts of included minutes and messages, different base prices, and different costs for going over the number of included minutes and messages. Companies carefully design their plans to earn the most amount of money, not necessarily the most number of customers. To do this, they analyze data from cell phone users and calculate averages to find a single value that is typical for different groups of customers. They also use linear functions to represent the relationships between the base price, included minutes, and overage rate to help decide how to set the amounts for each plan.