题目
Students of the same age from two schools, school A and school B, take a large number of quizzes throughout the year and are each awarded a mark out of 1000. The marks of 123 students in school A and 147 students in school B are ranked from lowest (rank 1) to highest (rank 270). The sum of the ranks of the students from school A is 15 355.
(a) Carry out a Wilcoxon rank-sum test at the significance level to investigate whether there is a difference in average marks between the students in school A and school B.
(b) State an assumption that is required for the Wilcoxon rank-sum test to be valid.
题目中文翻译
来自两所学校（school A 和 school B）的同龄学生全年参加大量小测，每名学生获得一个满分为 1000 的分数。school A 的 123 名学生和 school B 的 147 名学生的分数从低到高排名（最低为 rank 1，最高为 rank 270）。school A 学生的秩和为 15 355。
(a) 在 显著性水平下进行 Wilcoxon 秩和检验，研究 school A 和 school B 学生的平均分是否存在差异。
(b) 写出 Wilcoxon 秩和检验有效所需的一个假设。