3.. What should we do when we have the full population? Can we close the case and declare a definite answer? Long story in short: even if we have the number (e.g. test score) from every body, but each person still has within-subject variations. For example, if you retake a test (e.g. SAT, ACT, GRE), you might not get the same score. I took GRE three times and each time I got a different score. Do we REALLY have the FULL information? This is a very deep question and it is beyond the scope of this course.