The top plots are population distributions; the bottom two are sampling distributions of the sample mean. Match numbers to letters.
Say we want estimate the size of an average class at Berkeley.
Should we ask the administration? Or should we survey the students, ask them how large their classes are, and take the average of these values? Does it matter?
How would the sampling distribution change if instead of calling on the front row, the Prof. put all 527 names on tickets in a box, mixed them up, then drew 18 names without replacement?