Padmawar, V R2014-01-152014-01-151994Journal of Statistical Planning and Inference, v 40, p 81-95http://hdl.handle.net/10263/5765enSampling strategiesVariance estimatorsUnbiasednessDefinite matricesQuadratic formsUniform nonnegativityStrategies admitting nonnegative unbiased variance estimatorsArticle