The Trustees make basic assumptions for several of these factors based on analysis of historical trends, historical conditions, and expected future conditions. These factors include fertility, mortality, immigration, marriage, divorce, productivity, inflation, average earnings, unemployment, real interest rates, and disability
incidence and
termination. Other factors depend on these basic assumptions. These other, often interdependent, factors include total population, life expectancy, labor force participation,
gross domestic product, and program-specific factors. Each year the Trustees reexamine these assumptions and methods in light of new information and make appropriate revisions. The assumptions for this report were selected by the middle of January 2019.
Future levels of these factors and their interrelationships are inherently uncertain. To address these uncertainties, this report uses three sets of assumptions, designated as intermediate (alternative II),
low-cost (alternative I), and
high-cost (alternative III). The intermediate set represents the Trustees’ best estimate of the future course of the population and the economy. With regard to the net effect on the actuarial status of the OASDI program, the low-cost set is more optimistic and the high-cost set is more pessimistic. The low-cost and high-cost sets of assumptions reflect significant potential changes in the interrelationships among factors, as well as changes in the values for individual factors.
The following sections briefly discuss the various assumptions and methods used in making the estimates of trust fund actuarial status, which are the focus of this report.
1 There are, of course, many interrelationships among these factors that are important but are beyond the scope of this discussion.
Birth rates by single year of age, for women aged 14 to 49,
2 are the basis for the fertility assumptions. These rates apply to the total number of women, across all marital statuses, in the midyear population at each age. Table
V.A1 displays the historical and projected total fertility rates.
3 Historical death rates were calculated for years 1900 through 2016 for ages below 65 (and for all ages for years prior to 1968) using data from the National Center for Health Statistics (NCHS).
4 For ages 65 and over, final
Medicare data on deaths for years 1968 through 2015 and preliminary data for 2016 and 2017 were used.
5 Death rates by cause of death were produced for all ages for years 1979‑2016 using data from the NCHS.
The total age-sex-adjusted death rate
6 declined at an average annual rate of 1.03 percent between 1900 and 2016. Between 1979 and 2016, the period for which death rates were analyzed by cause, the total age-sex-adjusted death rate, for all causes combined, declined at an average rate of 0.87 percent per year.
The trends in the annual reductions in central death rates were calculated for the period from 2006 to 2016 for NCHS data, and 2007 to 2017 for Medicare data, by age group, sex, and cause of death.
7 These trends are the starting reductions for alternative II. For alternatives I and III, 50 and 150 percent of the starting reductions are used, respectively. These annual reductions, by alternative, are assumed to transition rapidly from the starting reductions until they reach the ultimate annual percentage reductions assumed for 2043 and later.
Table V.A1 contains historical and projected age-sex-adjusted death rates for the total population (all ages), for ages under 65, and for ages 65 and over. Age-sex adjustment eliminates the effect of a changing distribution of population by age and sex, allowing the pure effects of changes in death rates to be observed. Under the intermediate assumptions, projected age-sex-adjusted death rates are, in general, slightly higher than the death rates in last year’s report for both the age group under 65 and the age group 65 and over. These changes primarily result from incorporating more recent historical data, which continue to show low rates of improvement.
This report presents a July 1 (i.e., midyear) population for each year, which is derived from surrounding December populations. Table V.A3 shows the historical and projected population for July 1 by broad age group, for the three alternatives. It also shows the aged and total dependency ratios (see table footnotes for definitions).
Life expectancy at a given age reflects death rates at that and all older ages. Period life expectancy is somewhat related to the age-sex-adjusted death rate discussed in section V.A.2. However, life expectancy places far greater weight on death rates at relatively younger ages than those at relatively older ages. Therefore, changes in death rates at younger ages have far greater effects in changing life expectancy over time. It is important to keep this concept in mind when considering trends in life expectancy.
