Ononot — In a two-phase sampling design, suppose we have a primary sa

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In a two-phase sampling design, suppose we have a primary sample of size $n'$ and a subsample of size $n$ . The regression estimator for the population mean $\bar{Y}$ is $\bar{y}_{lr} = \bar{y} + b(\bar{X}' - \bar{x})$ . What is the approximate variance of this estimator, assuming $n$ and $n'$ are large?