Analysis of Children's Health Insurance Patterns: Findings from the SIPP. 3. Representation of Children Over Time

05/12/1999

Table 1 presents comparative estimates of the population of children represented by the SIPP panel sample and the population of children represented by the individual calendar month samples. The latter estimates were obtained by summing the calendar month weights by single year of age for selected months. These population totals represent, approximately, the Census Bureau’s estimates of the population that would have been eligible for selection into the SIPP sample in each of the individual months. Estimates of the populations represented by the panel and cross-section samples are compared at four points in time: January 1992, October 1992, September 1993, and September 1994. January 1992 is the common reference month for the four rotation groups in the first wave of the 1992 panel. The next three months represent the beginning, middle, and end of the two-year period defined by FY93 and FY94, or the period on which our analysis is focused.

The first thing to note in this table is that the population of children to which the SIPP panel “weights up” in January 1992 actually exceeds the size of the population implied by the calendar month weights--by about 1.1 million children. We have no explanation for a difference of this magnitude. While the SIPP panel is weighted to estimates of the relevant population in March rather than January and, therefore, would not be expected to reproduce the January 1992 population totals, neither would we expect it to exceed the January 1992 population counts, much less by such a large margin. With this discrepancy in 1992, and the opposite trends in the two series, the two estimates of children under 19 cross between January and October, 1992.

From January 1992 through September 1994, the SIPP panel estimate of the population of children declines by about 2.2 million while the population implied by the calendar month weights rises by 2.9 million. The decline in the SIPP estimates can be attributed to the SIPP panel’s underrepresentation of births, which propagates through the younger ages. In January 1992 the SIPP panel sample represents an estimated 4.5 million infants. By October of that year the number of infants has dropped by nearly 1.4 million to 3.1 million. The number rises some by September 1993 but then drops by half a million by September 1994. Because the children born into the panel in 1992 become the panel’s one-year-olds in 1993 and two-year-olds in 1994, the effect of the underrepresentation of births is compounded. By September 1994 the SIPP panel represents 3.5 million fewer children under the age of three than it does in January 1992. The Census Bureau’s population estimates reflected in the calendar month weights indicate that the size of this population did decline over this period, but by only 150,000.

TABLE 1: COMPARISON OF THE POPULATION OF CHILDREN UNDER 19 REPRESENTED BY THE 1992 SIPP PANELAND THE SIPP CROSS-SECTION SAMPLE: JANUARY 1992 TO SEPTEMBER 1994
  Estimates from the 1992 SIPP Panel Sample Estimates from the 1992 SIPPCross-section Sample
Age Jan 1992 Oct 1992 Sept 1993 Sept 1994 Jan 1992 Oct 1992 Sept 1993 Sept 1994
Total 72,103,000 71,549,000 70,868,000 69,935,000 71,016,000 72,374,000 73,137,000 73,957,000
0 4,490,000 3,123,000 3,469,000 2,943,000 4,214,000 4,058,000 4,082,000 3,984,000
1 4,236,000 4,569,000 3,008,000 3,392,000 4,048,000 4,168,000 4,016,000 4,127,000
2 4,109,000 4,084,000 4,558,000 2,992,000 4,010,000 4,173,000 4,102,000 4,010,000
3 3,969,000 4,006,000 4,043,000 4,501,000 3,910,000 3,964,000 4,192,000 4,176,000
4 4,033,000 3,880,000 4,004,000 4,021,000 3,899,000 3,886,000 3,962,000 4,152,000
5 3,629,000 4,054,000 3,867,000 3,975,000 3,871,000 3,934,000 4,003,000 3,899,000
6 3,854,000 3,598,000 3,993,000 3,854,000 3,795,000 3,790,000 3,876,000 4,068,000
7 3,768,000 3,759,000 3,607,000 3,988,000 3,757,000 3,852,000 3,886,000 3,961,000
8 3,583,000 3,759,000 3,841,000 3,581,000 3,577,000 3,562,000 3,805,000 3,683,000
9 3,814,000 3,641,000 3,532,000 3,837,000 3,912,000 3,888,000 3,787,000 4,015,000
10 3,816,000 3,841,000 3,785,000 3,519,000 3,814,000 3,968,000 3,722,000 3,774,000
11 4,058,000 3,838,000 3,742,000 3,785,000 3,893,000 3,782,000 4,009,000 3,927,000
12 3,584,000 4,045,000 3,931,000 3,736,000 3,616,000 3,863,000 3,723,000 3,793,000
13 3,471,000 3,458,000 3,906,000 3,919,000 3,507,000 3,576,000 3,841,000 3,861,000
14 3,534,000 3,477,000 3,461,000 3,886,000 3,542,000 3,540,000 3,680,000 3,883,000
15 3,748,000 3,449,000 3,376,000 3,443,000 3,517,000 3,628,000 3,620,000 3,713,000
16 3,778,000 3,776,000 3,578,000 3,370,000 3,577,000 3,631,000 3,707,000 3,617,000
17 3,532,000 3,612,000 3,678,000 3,551,000 3,331,000 3,454,000 3,515,000 3,790,000
18 3,095,000 3,580,000 3,490,000 3,642,000 3,226,000 3,656,000 3,609,000 3,523,000
Change from Jan 1992
Total   -554,000 -1,235,000 -2,168,000   1,358,000 2,121,000 2,941,000
0 to 2   -1,060,000 -1,800,000 -3,508,000   127,000 -72,000 -151,000
3 to 18   505,000 565,000 1,341,000   1,231,000 2,193,000 3,092,000
3 to 5   309,000 283,000 866,000   104,000 478,000 548,000
6 to 10   -238,000 -78,000 -56,000   206,000 221,000 647,000
11 to 15   -128,000 21,000 376,000   314,000 796,000 1,100,000
16 to 18   561,000 340,000 156,000   608,000 698,000 797,000

The net difference of 3.35 million between the two samples accounts for most of the 4.0 million children that would be eligible for SIPP in September 1994 but are not represented by the 1992 panel. The remaining .65 million is spread over the ages 3 through 18. The population estimates show this population growing by 3.1 million between January 1992 and September 1994 whereas the SIPP panel shows growth of 1.3 million. The population of children 3 through 18 grows in SIPP because the number of children who move into this age group from younger ages exceeds the number who “age out” at the upper end or leave the population through death, migration, or institutionalization. The SIPP panel estimates of children 3 to 5 grow by more than the Census Bureau’s population estimates because the SIPP panel overrepresents infants in January 1992. In the 6 to 10 age group, the SIPP panel declines slightly over time while the Census Bureau’s population estimates grow by nearly 650,000. In the 11 to 15 age group, the SIPP panel estimate increases by 376,000 while the population estimate rises by 1.1 million. Finally, in the 16 to 18 age group, the population represented by the SIPP panel grows by 156,000 compared to 797,000 for the cross-section sample..

That the differences between the growth trajectories of the SIPP panel and the total population (of SIPP-eligible children) are relatively small over most of the age range suggests that we can generalize from the SIPP panel to the full population fairly readily. It is only at the lower end of the age distribution that we need to be conscious of major differences between the sample and the population. Indeed, with the SIPP panel representing just over three-quarters of the estimated number of children under three in the population, we should be aware of the potential impact on the distribution of characteristics that differ substantially between very young children and older children.

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