Let $X \square \operatorname { Bin } \left( m , p _ { 1 } \right) \text { and } Y \square \operatorname { Bin } \left( n , p _ { 2 } \right)$

Essay

Let $X \square \operatorname { Bin } \left( m , p _ { 1 } \right) \text { and } Y \square \operatorname { Bin } \left( n , p _ { 2 } \right)$ with X and Y independent variables, and let $\hat { p } _ { 1 } = X / m \text { and } \hat { p } _ { 2 } = Y / \mathrm { n } \text {. Then } \mathrm { E } \left( \hat { \mathrm { p } } _ { 1 } - \hat { \mathrm { p } } _ { 2 } \right) =\underline{\quad\quad}$ ,so $\hat { p } _ { 1 } - \hat { p } _ { 2 }$ is an __________ estimator of $p _ { 1 } - p _ { 2 }$