claim experience to develop our claim resolution assumptions. These assumptions are established for the probability of death and the probability of recovery from disability. Our studies review actual claim resolution experience over a number of years, with more weight placed on our experience in the more recent years. We also consider any expected future changes in claim resolution experience. 3. The  incidence rate , used for policy reserves and IBNR claim reserves, is the rate at which new claims are submitted to us. The incidence rate is affected by many factors, including the age of the insured, the insured's occupation or industry, the benefit plan design, and certain external factors such as consumer confidence and levels of unemployment. We establish our incidence assumption using a historical review of actual incidence results along with an outlook of future incidence expectations. Establishing reserve assumptions is complex and involves many factors. Reserves, particularly for policies offering insurance coverage for long-term disabilities and long-term care, are dependent on numerous assumptions other than just those presented in the preceding discussion. The impact of internal and external events, such as changes in claims operational procedures, economic trends such as the rate of unemployment and the level of consumer confidence, the emergence of new diseases, new trends and developments in medical treatments, and legal trends and legislative changes, including changes to social security and other government-based welfare benefits programs which provide policy benefit offsets, among other factors, will influence claim incidence rates, claim resolution rates, and claim costs. In addition, for policies offering coverage for disability or long-term care at advanced ages, the level and pattern of mortality rates at advanced ages will impact overall benefit costs. Reserve assumptions differ by product line and by policy type within a product line. Additionally, in any period and over time, our actual experience may have a positive or negative variance from our long-term assumptions, either singularly or collectively, and these variances may offset each other. We test the overall adequacy of our reserves using all assumptions and with a long-term view of our expected experience over the life of a block of business rather than test just one or a few assumptions independently that may be aberrant over a short period of time. Therefore, while it is possible to evaluate the sensitivity of overall adequacy results in our reserves based upon a change in each individual assumption, the actual impacts of changes to a variety of underlying assumptions must be considered in the aggregate by product line in order to judge the overall potential implications to reserve adequacy. The following section presents an overview of our trend analysis for key assumptions and the results of variability in our assumptions, in aggregate, for the reserves which we believe are reasonably possible to have a material impact on our future financial results if actual claims yield a materially different amount than what we currently expect and have reserved for, either favorable or unfavorable. In December 2020, we reinsured approximately 75 percent of our Closed Block individual disability business pursuant to a reinsurance agreement with Commonwealth and expect that we will complete the second phase of this transaction in the first quarter of 2021 to cede a significant portion of the remaining business. As a result, we are no longer incorporating this block of business into our discussion of trends in key assumptions below. Generally, we do not expect our mortality and morbidity claim incidence trends or our persistency trends 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. We have historically experienced an increase in our group long-term disability morbidity claim incidence trends during and following a recessionary period and believe claim incidence trends may continue to somewhat follow general economic conditions and demographics of the general workforce. Regarding the 2020 COVID-19 pandemic, in the short-term we have experienced elevated mortality and anticipate continuation through part of 2021. However, at this time, our view on our long-term mortality and morbidity expectations has not been impacted by this limited experience. Claim incidence rates for Unum US group long-ter m disability were generally consistent in 2 020  compared to the prior year. In  2020, both short-term and long-term interest rates decreased. The long-term interest rates supporting the majority of our lines of business remain below historical norms. The assumptions we used to  discount reserves during this period were slightly lower for certain of our product lines. Reserve discount rate assumptions for new policies and new claims are periodically adjusted to reflect our current and expected net investment returns. Changes in our average discount rate assumptions tend to occur gradually over a longer period of time because of the long-duration investment portfolios which support the reserves for the majority of our lines of business. Trends in Key Assumptions Our claim resolution rate assumption used in determining reserves is our expectation of the resolution rate we will experience over the life of the block of business and will vary from actual experience in any one period, both favorably and unfavorably. Claim resolution rates are very sensitive to operational and environmental changes and have a greater chance of significant variability in a shorter period of time than our other reserve assumptions. These rates are reviewed on a quarterly basis for the death and recovery components separately. Claim resolution rates in our Unum US group long-term disability product line have 50