A school wishes to give an estimate of the average score of its sixth-grade students to a reading comprehension exam. The students of the school are grouped into three streams: the faster students being grouped in stream I and the slower students in stream III. The school decides to stratify in relation to streams. The sixth grade has 50 students in lane I, 90 in lane II and 60 in lane III. A stratified sample of 50 students is distributed proportionally in the sectors (we obtain nI = 14, nII = 20 and nIII = 16, respectively). The sample results are shown below:

Stream y S 79.71 105.14 II 64.75 158.20 III 37.44 186.13