Plikusas, A., and D. Pumputis. “Estimation of the Finite Population Covariance Using Calibration”. Nonlinear Analysis: Modelling and Control 15, no. 3 (July 25, 2010): 325–340. Accessed August 28, 2026. https://zurnalai.vu.lt/nonlinear-analysis/article/view/14328.