Final answer:
The rate of interest the annuity earns is the factor that does NOT directly impact the monthly amount of benefit an annuitant receives. Instead, the principal amount, length of payout period, and exclusion ratio play a direct role in determining the benefit amount.
Step-by-step explanation:
The monthly amount of benefit an annuitant receives is based on several factors, but not all of them. The factors that are indeed part of the calculation include the principal amount, the length of the payout period, and the exclusion ratio applied. The exclusion ratio is used to determine what portion of the annuity payments will not be taxed. However, the factor that does not directly affect the monthly benefit amount is the rate of interest the annuity earns.
While this rate can impact the underlying value of the annuity and the amount of money the annuity can earn over time, it does not directly determine the monthly payment amount, which is typically fixed in the case of defined benefit plans, such as private company pensions described as providing fixed annual payments or 'defined benefits' to retirees.