The Graybill–Deal estimator is often used to estimate the common mean of two normal populations with unknown and possibly unequal…

The Graybill–Deal estimator is often used to estimate the common mean of two normal populations with unknown and possibly unequal variances. Though this estimator is generally unbiased, its admissibility remains to be shown.

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Source: List of unsolved problems in statistics (CC BY-SA 4.0)