Robert, a rookie statistician, decides he wants to practice the methods he learned for performing a proportion -test in order to determine if the average height of children at a local elementary school has increased over time. Last year every child was measured and the mean height of all students was determined to be 42 inches. This year he sampled the first six students he noticed that were wearing light-up shoes. What are the three mistakes Robert has made so far while performing his research? What steps would you take to improve his research