Ononot — When applying the Horvitz-Thompson estimator \hat{Y}{HT} = \

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Data Collectionhard

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When applying the Horvitz-Thompson estimator $\hat{Y}_{HT} = \sum_{i \in s} \frac{y_i}{\pi_i}$ for unequal probability sampling without replacement, what condition on the joint inclusion probabilities $\pi_{ij}$ is required to calculate an unbiased estimator for the variance $V(\hat{Y}_{HT})$ ?