Following 'A system of model fertility schedules with graphically intuitive parameters' by Carl Schmertmann (2003)
Via https://www.demographic-research.org/volumes/vol9/5/
Including its Appendix B: Non-matrix calculation of spline coefficients
And additional Excel file via https://www.demographic-research.org/volumes/vol9/5/files/FILE%20I%20QS%20Model.xls
Related Shiny for R application by Carl Schmertmann, for teaching and understanding the model: https://schmertmann.shinyapps.io/QS-interactive/
-August 2021