A sample mean is an unbiased estimator of the population mean. In other words, the expected difference between the population mean and the sample mean is zero regardless of the population distribution. In other words $E[\bar x - x_p]=0$, where $\bar x$ and $x_p$ are the sample and population mean, respectively.
Given that the population is normally distributed with variance $\sigma^2$ and knowing the sample size $n$, what is the expected absolute difference between the population mean and the sample mean?
or in mathematical form:
$$E[\space| \bar x - x_p |\space] = \space ?$$
The vertical lines stands for "absolute value"