Five formulas of spline interpolation and their applications to demographic data are introduced.  "The formulas are applied to (1) interpolating and smoothing values of life table functions....(2) interpolating births by five-year age groups of mothers into single ages....(3) interpolating cause-specific deaths by five-year age groups into single ages....[and] (4) interpolating and smoothing two-dimensional demographic data e.g. mortality rates and expectations of life by year and age." (SUMMARY IN ENG)