So far, we have been testing for a difference that
can occur on both sides of the standard normal
distribution
In our example, the z-value of 2.78 gave a P-value
of 0.0054
For a one-tailed test, a z-score of 2.78 will give a
P-value of 0.0027 (=0.0054/2). That is occurred
when we are absolutely sure that the mean birth
weight of our sample always exceeds that of the
general population
