Date: Mon, 7 Jul 2008 17:20:12 -0700 Reply-To: jenmoocat Sender: "SAS(r) Discussion" From: jenmoocat Organization: http://groups.google.com Subject: PROC MDS using preference rankings Comments: To: sas-l@uga.edu Content-Type: text/plain; charset=ISO-8859-1 Ever have one of those days where you feel like you are just missing something? Hopefully someone can help -- because I am beginning to feel like an idiot. I have preference rankings for 6 brands for two time periods (t=0 and t=1). I would like to overlay the preference map from time t=0 with the map from time t=1, so we can easily see how preferences have changed over time. My data looks like the following (for one time period and 5000 respondents): respondent brand1 brand2 brand3 brand4 brand5 brand 6 1 5 2 1 2 3 5 2 4 3 3 3 3 3 3 2 2 5 1 1 1 4 1 2 4 4 4 4 ... ... ... 5000 2 5 5 3 2 1 And I have another dataset for the next time period --- with changed rankings (hopefully). I've spent the past couple of days reading the PROC MDS documentation pages and the MDS chapter in "Multivariate Data Analysis" by Hair, et al., and searching around the web --- and I am stymied. Although the textbook talks about being about to use preference rankings, all examples that I've seen (in the book and on the web) have been based on starting with a similarity matrix. And I just don't quite grok the best way to go from my preference ranking data to a similarity matrix. Or whether PROC MDS can be used with raw ranking data, instead of the matrices. Should the similarity matrix just simply be constructed using the following steps: for each respondent, calculate a matrix that contains the simple numerical difference between the rankings: so for respondent 1, it would look something like: brand1 brand2 brand3 brand4 brand5 brand6 brand1 0 3 4 3 2 0 brand2 -3 0 1 0 -1 -3 brand3 -4 -1 0 -1 -2 -4 brand4 -3 0 1 0 -1 -3 brand5 -2 1 2 1 0 -2 brand6 0 3 4 3 2 0 then either 1) create one "similarity matrix" by taking the averages over all responders or 2) stack these matrices, one responder over another (and use the CONDITION=ROWS option) Am I on the right track? I feel like I am just missing something obvious and am at the point where I feel like banging my head against the wall. Can anyone shed a little light on this for me? Thanks muchly in advance, -jennifer

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