Posted by **Colin** on Thursday, August 13, 2009 at 5:21am.

I have 28 participants. Each completed five tests. The maximum score a participant could receive on any one test was 4/4. The lowest score anyone could receive was zero.

I will do a Friedman test for this paired data.

My question: given that I have a very large number of identical ranks (mostly participants getting zero in all tests) can I still go ahead? I just want to know if there is a significant difference in the medians.

Thanks!

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**bobpursley**, Thursday, August 13, 2009 at 8:34am
Yes, assuming the zero was a valid score, as the person being tested didn't know the subject matter. If the zero is a zero for other reasons (absent, being hardheaded and refusing the test, or similar), then the conclusion on difference is likely to be invalid.

I am reminded of instances were kids put their name on answer sheets, and turn it in as an act of rebellion. You can't make a conclusion about the meaning of scores in that case.

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**Colin**, Thursday, August 13, 2009 at 9:53am
Thanks very much Bob. I much appreciate your taking the trouble to respond.

All the best!

Colin

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**SraJMcGin**, Thursday, August 13, 2009 at 10:02am
After reading Bob's comment, I can't help but remember the best French student I had purposely getting himself a zero but not only putting a wrong answer but the very worst answer! The principal was so angry he wanted to suspend that student but instead I persuaded him to just delete that score, because I knew why he had done that! I'm happy to say that student went on to be a wonderful surgeon.

Sra (aka Mme)

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