Appendix D: Construction of Quarterly Interpolating Series for U.S. Department of Commerce Annual Import Price Indexes

Robert E. Lipsey, Price and Quantity Trends in the Foreign Trade of the United States (Princeton University Press, 1963), https://www.nber.org/books-and-chapters/price-and-quantity-trends-foreign-trade-united-states/appendix-d-construction-quarterly-interpolating-series-us-department-commerce-annual-import-price.