Cubic spline

Cubic splines help us draw a smooth line through lots of data points, like death rates at each age. Instead of connecting the dots with straight lines (which can look jagged), a cubic spline creates a gentle curve that follows the overall shape of the data. This makes it easier to see real patterns and changes in how death rates increase with age, rather than being distracted by random bumps or drops. In longevity modeling, this helps experts make better predictions about how long people might live, because the curve gives a clearer, more realistic picture of underlying mortality rates.