Seem no yuav siv sij hawm ib tug zauv mus kom ze rau piv txwv thiab kwv yees nyob rau hauv ntau yam thiab uas tsis yog-yuav qhov muaj tseeb kuaj. Nws yuav kos rau Särndal, Swensson, and Wretman (2003) thiab Särndal and Lundström (2005) .