SINGH, H. P.; GANGELE, R. K. Classes of almost unbiased ratio and product type estimators in two phase sampling. Statistica, [S. l.], v. 59, n. 1, p. 109–124, 1999. DOI: 10.6092/issn.1973-2201/1107. Disponível em: https://rivista-statistica.unibo.it/article/view/1107. Acesso em: 17 oct. 2021.