We believe that these ranges provide a reasonable estimate of the possible changes in reserve balances for those product lines where we believe it is possible that variability in the assumptions, in the aggregate, could result in a material impact on our reserve levels, but we record our reserves based on our long-term best estimate. Because these product lines have long-term claim payout periods, there is a greater potential for significant variability in claim costs, either positive or negative. We closely monitor emerging experience and use these results to inform our view of long-term assumptions. We defer incremental direct costs associated with the successful acquisition of new or renewal insurance contracts and amortize these costs over the life of the related policies. Deferred costs include certain commissions, other agency compensation, selection and policy issue expenses, and field expenses. Acquisition costs that do not vary with the production of new business, such as commissions on group products which are generally level throughout the life of the policy, are excluded from deferral. Approximately 93.4 percent of our DAC relates to non-interest sensitive products, and we amortize DAC for these products in proportion to the premium income we expect to receive over the life of the policies. DAC related to interest sensitive policies is amortized over the lives of the policies in relation to the present value of estimated gross profits from surrender charges, mortality margins, investment returns, and expense margins. Key assumptions used in developing the future amortization of DAC are persistency, premium income, and for our interest sensitive products, mortality margins and investment returns. We use our own historical experience and expectation of the future performance of our businesses in determining our assumptions. For non-interest sensitive products, the estimated premium income in the early years of the amortization period is generally higher than in the later years due to the anticipated cumulative effect of policy persistency in the early years, which results in a greater proportion of the costs being amortized in the early years of the life of the policy. Our key assumptions used to develop the future amortization of acquisition costs deferred during 2020 did not change materially from those used in 2019. Generally, we do not expect our key assumptions to change significantly in the short-term, and to the extent that these trends do change, we expect those changes to be gradual over a longer period of time. Deferred Acquisition Costs (DAC) 52