Average squared error - MSE : Mean Squared Error
It allows for the definition of the quality of the second-degree estimator's quality to be defined.
\(MSE\left(\hat{\theta}\right)\equiv E\left[\left(\hat{\theta}-\theta^\mathrm{2}\right)\right]=Var\left[\hat{\theta}\right]-b\left(\hat{\theta}\right)^\mathrm{2}\) (3)
The smaller an estimator's MSE, the more accurate it is.