For X1,…,Xn∼Poisson(λ)X_1, \dots, X_n \sim \text{Poisson}(\lambda)X1,…,Xn∼Poisson(λ) (i.i.d.), which is unbiased for λ2\lambda^2λ2?
Xˉ2\bar{X}^2Xˉ2
Xˉ\bar{X}Xˉ
Xˉ2−Xˉn\bar{X}^2 - \dfrac{\bar{X}}{n}Xˉ2−nXˉ
Xˉ2−Xˉ\bar{X}^2 - \bar{X}Xˉ2−Xˉ
