The average is measure of central tendency that is also called sample mean and is calculated from the following formula:
This is an estimator of the population mean and is unbiased since the expected value for the estimator is the same parameter as we can see here:
And is important because is the most common measure of central tendency reported on any study.
Is important to remember that this measure can be affected by outliers , for this case when we have outliers is better use the median as a measure of central tendency
Not a function because of the vertical line test. Domain is [-2,∞] since the left most point is -2 and the right is unbounded. Range is (-∞,∞) since it is not bounded in terms of y.