{"id":596,"date":"2015-07-20T08:19:52","date_gmt":"2015-07-20T12:19:52","guid":{"rendered":"http:\/\/sites.berry.edu\/vbissonnette\/?page_id=596"},"modified":"2016-06-24T07:42:56","modified_gmt":"2016-06-24T11:42:56","slug":"mann-whitney-u-test","status":"publish","type":"page","link":"https:\/\/sites.berry.edu\/vbissonnette\/index\/stats-homework\/documentation\/mann-whitney-u-test\/","title":{"rendered":"Mann-Whitney U Test"},"content":{"rendered":"

Example homework problem:<\/h3>\n

Twenty participants were given a list of 20 words to process. The 20 participants were randomly assigned to one of two treatment conditions. Half were instructed to count the number of vowels in each word (shallow processing). Half were instructed to judge whether the object described by each word would be useful if one were stranded on a desert island (deep processing). After a brief distractor task, all subjects were given a surprise free recall task. The number of words correctly recalled was recorded for each subject. Here are the data:<\/p>\n\n\n\n\n
Shallow Processing:<\/td>\n13<\/td>\n12<\/td>\n11<\/td>\n9<\/td>\n11<\/td>\n13<\/td>\n14<\/td>\n14<\/td>\n14<\/td>\n15<\/td>\n<\/tr>\n
Deep Processing:<\/td>\n12<\/td>\n15<\/td>\n14<\/td>\n14<\/td>\n13<\/td>\n12<\/td>\n15<\/td>\n14<\/td>\n16<\/td>\n17<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Did the instructions given to the participants significantly affect their level of recall (alpha = .05)?<\/p>\n

Note that these are the same data that we worked with when you were working\u00a0 with the independent t<\/i> test procedure<\/a>. This will allow you to compare and contrast the results of the two procedures.<\/p>\n

\n

Enter these data into the first two columns of Stats Homework’s\u00a0<\/i>data manager and rename the variables. Your screen should look like this:<\/p>\n

\"ittest9\"<\/a><\/p>\n

Make sure to double-check and save your data. To conduct your analysis,\u00a0pull down the Analyze<\/b> menu, choose Non-Parametric Tests<\/b>, and then choose Wilcoxon W \/ Mann-Whitney U<\/b>.<\/p>\n

\"mwu1\"<\/a><\/p>\n

Before we look at the output produced by this procedure, let’s take a look at the variables that have been created by it and written back into your data:<\/p>\n

\"mwu4\"<\/a><\/p>\n

Note that Stats Homework<\/i> has ranked all of your scores from 1 to 20.\u00a0Then, it produced two variables: the ranks for your first group\/variable (Rank_1), and the ranks for your second group\/variable (Rank_2). \u00a0Both the Wilcoxon W test and the Mann-Whitney U test are computed from the sums of these ranks.<\/p>\n

Basic Output<\/h4>\n

\"mwu2\"<\/a><\/p>\n

Descriptive Statistics<\/b>. This table includes the number of scores
\nfor each group, and the sum of the ranks for each group.<\/p>\n

Exact Test<\/b>. This table includes the non-parametric test statistics and
\nan exact test of their significance level.<\/p>\n